That's why I say LATE Model A...late ones and B's have a rear bushing, earlier a bearing back there, and there are other differences. Just go to a supplier who lists everything for the A's, and where there's a difference be sure to choose one listed for '31...Bratton is begiining to note parts that interchange with B, an advantage. Armature also has slight differences between versions. late is again same as B.
There are different end plates (more than one design) used in B's, and the little tab that connects to cutout on A's is replaced by a wire, but generally all the stuff you need for an overhaul is the same for 1931 A's...meaning almost all parts are readily available.