My friend John Cochran put me in touch with a very nice chap in Scotland named Ian Caldwell who happened to have a Model B gearbox for sale. The Model B gearbox offers the advantage of synchromesh over the Model A by removing the need to double declutch.
In a synchromesh gearbox, to correctly match the speed of the gear to that of the shaft as the gear is engaged the collar initially applies a force to a cone-shaped brass clutch attached to the gear, which brings the speeds to match prior to the collar locking into place.
Now I still need to gather quite a lot of extra parts to effect the conversion, these include a clutch pedal, shift lever. I’ll post the progress!