Can I narrow the scope of your question? Looking just at the years with Eaton rears?
The Eaton rear was under all '47 and later 1-ton GM trucks up into the 60's, but by the end of the 60's Dana rears were used under at least some 3/4-ton trucks and may also have appeared under some 1-tons. Sometime in the 70's the GM 14-bolt replaced the Eaton, and though they have some similarities, they are a different rear. And at some point many 1-ton trucks came with the Dana 70HD including my own '89 K3500.
So I'm going to assume your question was just about GM trucks with the Eaton rear.
The spring pads welded on a '61 1-ton Eaton are wider and possibly positioned differently than on a '55 1st 1-ton Eaton. I've seen this difference with my own eyes and based upon it would imagine that the spring pads may be a little different in each series of trucks.
Another area of difference is the brakes. Early rears have Huck brakes and later ones have Bendix. I've heard enough about the problems in switching that I'd suggest getting a rear with the brakes you want. Aparently both the housing and the axle shafts are different between these two types of brakes.
I doubt this is an exhaustive list of all the differences, but just the items I could list mean the quick answer is no, they are not all the same.
If you can be more specific it may be that I'm listing differences that don't matter to you.
