Introduced in 1902 the Drummond flat-bed 3.5" x 16" (and rare 20-inch version) was, in its various guises, to continue in production for almost half a century. Although often called the "B-Type" that designation did not, strictly speaking, apply until the appearance of a much-modified lathe in 1912 and after the introduction of the Round Bed, in 1908, which Drummonds called the "A-Type". However, the maker's catalogue description, "31/2-inch Centre Back Geared, Self-acting Sliding, Boring, and Screwcutting Lathe" whilst accurate, is a little long for handy use, so for the purposes of easy reference to the very first flat-bed lathes we might call them "Pre B-Type Flat Beds".
It was certainly a well-thought out machine, made of good-quality materials and obviously designed by engineers familiar with the needs of their intended market; in many respects it incorporated refinements and features previously only found on much larger and more expensive lathes. Instead of a direct connection between changewheels and leadscrew the drive was first taken through "power shaft", with a dog clutch at its headstock end, that ran the length of the bed and engaged a gear at the tailstock end to transmit its drive up to the leadscrew proper that sat between the bed ways. This arrangement had the advantage of allowing the leadscrew, as on many very high-class toolroom lathes of later years, to transmit its pull to the carriage almost directly under the middle of the saddle. One very rare model, a long-bed version, incorporated an extra support foot towards the tailstock end. Because the bed was too long for the operator to comfortably turn the normal carriage drive handle (at the end of the bed), the support foot incorporated a worm-and-wheel drive that engaged with the powershaft.
Several changes were made in the first years of production to the headstock casting and its bearings: on the firtst model the whole unit was decidedly flimsy, with cast-in bracing on the horizontal face between the bearing uprights and each bearing pinched by a bolt that passed beneath it - an identical arrangement can be seen in the pictures of the company's contemporary (and very rare) circa 1900 4-inch model . The bed was also of minimal proportions - and so slight in depth at the tailstock end that a good deal of the central drive shaft beneath the leadscrew was exposed. However, this first design did not last long and as early as 1903/4 the headstock casting had been strengthened and the bearings changed to a type with a taper on the outside face and a screwed ring and serrated nut on the end by which means they could be pulled into the headstock casting - and so closed down to set the running clearance on the 3/4" x 8 t.p.i. spindle. Unfortunately this beautifully-engineered arrangement also failed to survive and, by 1905/6, Drummond had cheapened the assembly to allow the bearings to be drawn in by through-screws that pulled on thick steel washers; this design, though prone to chatter if not perfectly adjusted, lasted until the introduction of the "M-Type" in 1921. Although most pre-1912 lathes had a 3-step headstock pulley for drive by a 1/4" round leather (gut) drive examples are known of machines with 2-step pulleys that took a much larger belt; though not mentioned as an option in any known catalogue, these lathes would undoubtedly have been intended for tackling heavier work.
There was no compound slide; instead the swivelling tool slide could be locked by a single bolt into one of two T slots on the saddle (which doubled as a small but useful boring table) and slid into the most convenient position for the job in hand; it was even possible to swing it right round and use it as a rear tool post for parting off with an inverted tool - a decided improvement on trying to do the job from the front
In 1905 Drummond moved from their original Pinks Hill works to Rydes Hill - a change that was reflected in the company's machine name plates; however, knowing from Drummond from retired-piece work operatives how parsimony , it is likely that any unused badges would have been used up first so this handy dating pointer can only be considered approximate.
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The earliest type of flat-bed 3.3" centre height Drummond had a distinctive headstock with pinch bolts passing beneath the bearings and cast-in bracing struts on the face between the bearing uprights. The first beds were of light construction with the short section of underside at the headstock end almost parallel to the ways and at the tailstock end rising so close to the top surface that it exposed, in side elevation, a good deal of the power shaft that lay beneath the leadscrew.

Continued:
In 1905/6, having presumably settled into their new premises, further changes were made to the design and the maker's publicity material made much of both the new headstock spindle bearings (previously explained) and the improved bed casting that, being thicker and more heavily constructed, was considerably strengthened in comparison to the rather flimsy original and had a distinctive downward curls at the headstock end and a rather deeper wall at the tailstock. The first beds were very light and finished with their underside edge parallel to the ways at the headstock end and, at the other, rising in a graceful arch to become very close to the top surface (exposing both top and bottom surfaces the shaft drive beneath the leadscrew) before looping down to form a vertical flat mounting surface for the leadscrew bracket. These features are an easy way of determining whether a lathe is from one of the first production batches for even if early owners modified and upgraded other components they would have been unlikely to changed the bed.
Sadly, the elegant hardwood (or horn) knob on the backgear shaft disappeared to be replaced by a tapered, one-piece steel handle - and an altered design of leadscrew clutch, with two positions instead of three, was introduced. The tailstock was modified with a longer bracket extending from the base casting and the loose clamp, instead of being machined from a block of steel, was now made of cast iron; unfortunately, the convenient locking handle was omitted and replaced by a nut that required the application of a loose spanner. The simple knock-in barrel-guide pin was moved from the side of the casting to the top; photographs of a circa-1906 model with some of these features can be seen here.
Changewheels ran directly on plain studs and were retained by circular, knurled-edge nuts. The mounting  bracket was shaped like a letter "L", a design foible that made it difficult to set up a very fine feed to the carriage drive. Changewheels supplied as standard consisted of: 20t, 30t, 35t, 38t, 40t, 45t, 50t, 55t, 60t and 65t. To generate metric threads an additional 63t changewheel could be purchased at extra cost.
In about 1907 the largely ornamental bracket beneath the leadscrew dog-clutch handle was omitted (it was held in place by two square-headed bolts) and the single, swivelling top slide modified to reduce the bending moment on the assembly by bringing the clamping stud and nut in line with its slideway; as a final touch the base of the slide was given a set of inscribed degree graduations. It was 1908 before a compound slide rest was introduced - initially with round handwheels and then with the "cranked" type, a design that was to become a Drummond 'trade-mark'.  Although some of the above changes can be found in advertisements of the time, many cannot and numbers of lathes have been found where the specifications overlap and the year of production is uncertain - for example: lathes with a graduated base to the top slides, but still with the dog-clutch lever bracket in place; or a compound slide rest on a lathe with the older-style bearings - and it is entirely possible that the factory used up obsolete parts before a modification became permanent, or customers ordered a cheaper, alternative specification, took them back to the factory for repair, alteration or updating - or rebuilt and adapted them over the years using whatever later parts came to hand.
The headstock casting of all models could be swivelled on the bed and this movement, combined with that of the fully-adjustable tool slide, allowed the easy cutting of short tapers and tapered threads; however, it took until 1912 before the factory realised that fitting a locating dowel would help their customers in the important task of return the setting to zero. The tailstock could also be set over for long, fine-taper work. The gap in the bed could accommodate a piece of material a little over 9.5" in diameter and 3" thick. Supplied with the lathe as standard were: a faceplate, a small-angle plate with vertical T slots, catchplate, travelling steady, hand rest, two No. 1 Morse taper centres, a drive belt and spanners.
Although usually illustrated mounted on a self-contained treadle-and-flywheel stand, various kinds of wall and ceiling-mounted countershaft units were also available for those lucky enough to have either a stationary engine or even (though at great cost) an electricity supply and an electric motor. Although in its first incarnation this was a thoroughly well-specified product, it was substantially improved in 1912 and reached its final version through the two major rethinks of 1921 and 1925. It continued in production until 1942 when, overwhelmed with war-time demand for their high-speed production lathes, Drummonds were forced by the Ministry of Supply (who controlled all material and machine-tool allocations) to pass the rights over to Myford. Advertised as the "Myford M-Type" it became known in model engineering circles as the "Myford-Drummond M-Type" - a general history of the machine can be followed on the Drummond/Myford M Page.
A set of detailed photographs of a particularly fine and original early 31/2" lathe can be seen here and another lathe, still in regular use in Germany (and with just two owners) here.

The same early model as in the photograph above but suspected to have come from the second-edition catalogue showing a heavily art-worked picture disguising the change in the headstock casting and bearings--but not the bed  which is shown as the earliest type with the short length of underside at the headstock end finished in a section parallel to the ways and at the tailstock end rising to become very close to the ways before looping down to form a flat vertical surface for the leadscrew bracket. It is likely that the new headstocks were fitted for a while to earlier pattern beds--though for how long is impossible to say.
The simple, wooden packing pieces being used to mount the motor-cycle cylinder might seem an amateurish effort but was (and remains) a highly effective way of holding a fragile component without damage. The pin that limited the travel of the dog-clutch lever (it can be seen protruding through the bed immediately below the gap) was quickly dropped - as was the unnecessary support plate itself in 1907.

A 1907/8 model showing the optimum metal removal capability when using the proper tool on the right material at the correct speed - and with a delicate rate of feed, skilfully applied. Although most pre-1912 lathes had a 3-step headstock pulley for drive by a 1/4" round leather (gut) drive examples are known of machines with 2-step pulleys that took a much larger belt; though not mentioned as an option in any known catalogue, these lathes would undoubtedly have been intended for tackling heavier work.

Here the headstock is seen set over and a short taper being turned. The round leather rope drive from the rim of the foot-operated treadle drive allowed for a certain degree of twist in its drive. The sliding backgear was fitted with an elegant horn or hardwood handle.

Shown below is a 1908 Model lathe with the new-for-that-year compound slide rest - note that the "support plate" bracket under the leadscrew dog-clutch lever is no longer fitted and the more substantial bed in comparison with the machines illustrated above - most obvious being the increase in depth between the bed ways and the top of the radius at the tailstock end and the down sweep of the casting at the headstock end.