Originally Posted by 38 coupe
Now that I look more at the pictures of your generator, I think the whole generator is 1955. The back of the armature is smaller diameter to fit into the rear bearing; armatures going into bushings are larger diameter. The front and back plates include mounting ears. Originally H six cylinders used the same generator as the 8BA V8s. There was a separate bracket with a strap that held the original generator. Check out page 139 of the illustration section of the 48-56 Ford Truck Parts book (page 974 in the PDF). You can see the bracket and generator in this picture I found on Photobucket:
Interesting observation. I, too, thought the only difference in the generator could not possibly be just the back plate because the OD of the rear of the armature has to match the size of the bearing, or bushing. In the stock generator for 51, the rear bushing supports the armature. In my generator, the rear of the armature is supported by a bearing. After I discovered I have a later model rear plate with different brushes and a bearing instead of a bushing, I wondered if the generator was original. So what you say makes sense. I do not have a "mounting" band around my generator. But I have three bolts to secure it to the engine.