As far as I know, the essential structure of the pre-unit gearbox remained the same from the earliest Speedtwin to the 1962 650's, and that the only changes were to superficial external casting features. Indeed almost all of the essential internal engineering remained largely unchanged right up to the end of 750 twin production (such that the 5-speed clusters can be fitted to the pre-unit box) and even much of the Trident/Rocket3 is much the same.
I believe the pre-unit drain plug has always been located in the lower central casting, just above the bottom mounting, though I think that the pre-slickshift type box (pre-56) had the drain to the rear, whereas it is on the front with the 56-on slickshift type (presumably for the same different leaning reasons as the level plug change).
The main case went through one major change (other than the minor change noted above) between rigid frame and swingarm frame versions, in that the upper mounting was moved from the top-front of the casting (for the rigid) to top-centre of the casting (for the swingarm).
This happened at the same time as the chaicases went from long (for rigid) to short (for swingarm).
All the other changes were to the inner and outer covers, and some of these changes I would strongly suspect were made pre-slickshift. Some of the changes quite possibly happened during the brief overlap between rigid and swingarm frames for various models, so both bases would be covered.
The inner cover for the rigid (and the early swingarm) gearbox had a cast boss for the clutch cable, the cable pulling a vertical lever pivoted in the outer cover, which has an angled fill plug.
When the molds for the rigid version of the inner cover were made, I don't imagine their crystal ball showed them that the gearbox would be mounted at a different angle in years to come, so the only level boss was at the high rear position.
When the swingarm bike was introduced, I can imagine the existing mold being modified in a small way (it is simple because it is removing material from the mold, not adding to it) to produce the lower boss.
The cover could then be machined to suit either application at that era. This soon became irrelevant, as the models became entirely swingarm, but the old rigid boss remained in the casting till the end of the pre-units (though not drilled). It's like an evlutionary vestige of pre-history.
This arrangement persisted until the slickshift superceded it. The ouer cover was then changed considerably, to place the clutch lever moving horizontally on the top, the end of the cable now held in a special drawbolt, the filler moved to the top of the inner cover, and the distinctive chrome cover with 2 screws on the outer cover.
I don't know how many of these actually had the slickshift knob inside it. Was it meant for lazy riders or for sporting ones? It wasn't present in my 6T gearbox when I got it in 79, so I've not had the luxury of seeing how it worked.
It is hard to see how the speedo drive could ever be oil tight in the long term, but challenging it with high oil level is an invitation to a puddle or oil getting up into the speedo head.