I don’t know that I can give you a satisfactory answer. But generally speaking, there were 3 main types of armature plates during that era.

1. Those that had a groove around the outer edge, that the flywheel ran in. I understand it was a fire stop to avoid blowing the hood off motors with totally enclosed powerheads (Lower cowls).

2. Those that do not have the groove, such as the one in your picture. These were used in earlier models such as your 1953 that did not have lower cowls. They also had those big holes under the coils.

3. Further subdivisions include plates from the above two groups that did or did not utilize a wave (friction) washer in the mounting rings, These depend on whether or not it had a sliding throttle control on the front, such as the 3hp JW &.Lightwins.

I THINK the later ones with the groove were casting number 580210. Repeat “think” they were.