Although far from being expert in the AC Ace-Bristol I have read the original handbook for that model, and the AC Cars Ltd published workshop manual for the Bristol engines and gearboxes, both of which are held in the Archive. As far as I can tell:
a. Throughout, the handbook defines only the 100D2 engine, and includes no designation for the gearbox type;
b. The workshop manual cover defines the 100C, 100D and 100D2 engines, and the BWCR9 and BWCR12 gearboxes. Internally, the workshop manual defines the 100 D2 engine, but not the gearbox type. Nor is there any differentiation in the instructions therein between each type of engine and/or gearbox.
c. For completeness, the AC Cars handbook for the Greyhound does show different gearboxes for the 100D2 and 110 Bristol engines but, again, does not show the gearbox type.
Re the speedometer, it is not clear whether the inaccuracy you mention is simply down to kmph v mph or something more fundamental. At great risk of stating the obvious:
a. The optional final drive/rear axle ratios offered by AC Cars will make a pro-rata difference to speedo accuracy and I would expect each optional rear axle ratio to have a specific speedo drive gear ratio. No data have been found about this in either the handbook or the manual.
b. Any change in rear tyre section from the 1950s standard section could affect speedo accuracy. It is the rolling radius of the wheel/tyre combination which is of interest.
c. For any one axle ratio, for converting from a kmph speedo to one displaying mph is it not the case that only the speedo dial face is different?
Sorry, not a great deal of help!!
Good luck
John