Albion made great play in their November 1930 adverts for that years Motorcycle Show that they now made heavyweight gearboxes and three and four speed boxes for all types of engines were now available so in essence they started to appear on 1931 ranges. Your understanding as to when they were introduced is correct.
The three speed and reverse box came out at the same time.
It is easy to identify the general type of Albion box you have as they used a letter prefix system but identifying exactly what was fitted to what will drive you nuts. (Its driven me nuts for years.) For example in 1930 Albion produced 11 different models of gearbox. These could be had with either top or bottom platform mounts, a pivoting mount and also, using a longer mainshaft, could have different chain lines.
Plus of course the position of the platform mount was not fixed and could move across the box. (That one caused me considerable grief as I only realised when trying to install a replacement box and wondered why the chains didn't line up.)
Finally I resolved my problem by making a wooden jig with chain lines drawn on it. I took this to swapmeets with me so I could pop any Albion box on it and see whether it would fit my bike.
As you can gather, so far I haven't been able to discover any Albion marking code that makes sense to me beyond the general letter prefix and we haven't even got to clutches yet! If you find one I'll be first in the queue to congratulate you.
Cheers,
