Original early M springs had an extra coil and do not fit the later M and B boxes but the plunger gearbox springs fit the early boxes. The springs in the plunger boxes are of course interchangeable. Not sure about the '46 - '48 B boxes though.
I've just re-read your post, I'm not familiar with the B/A series boxes but I doubt that the M springs are compatible.
Last edited by MattL; 06/05/21 12:09 am.