I am rebuilding the rear axle, HO52. It is in a 1955.1 - 3600.I found on the plate end of the axle GM 77 on the short shaft and GM 78 on the other shaft. Do these numbers represent a 1955 axle or do they refer to something else? Check out the attachment for details.

Hy 87GN, those numbers probably are nothing more than numbers to identify the two different axles from each other so they can be assembled correctly into the housing in the factory.