Alas no idea on what the B means on the end model number - my guess would be a revision to the original, although looking at a parts list for a similar board (189) the letters used are A, B, M and T with no obvious difference.
If it's a read across from the main number, the letters seem to mean different things on different regulators, so 37A indicates ammeter terminals on a 37, while 155A indicates compensated voltage on the 155.
On the 75 (which will be popular with the softskin fans on the forum) 75B indicates for use with dynamo without field resistance, and 75BT (as on Bedford, Ford etc.) is the same but with a special low temperature regulator.