Now I'm less sure what your rear axle is. I was thinking it might be 10 lug which often uses 5 of those same bolt pattern on the front in GM applications.. but that's not what you have.
Some 40's GMC's I think used 5 lug which might be what you have, and it may be more like a 1 ton axle. 48-53 GMC 1.5 ton sometimes had 5 lug "Motor wheels" which might be what you have. If 20" rear tires I'd suspect an axle larger than 1 ton.
In any case it is not likely original to the 51 GMC you have. Might not even be from a GM truck. Post some more pictures of the axle and perhaps someone will know more. I think you can post more pictures in a reply.
1 ton and 3/4 ton front axles for 48-54 are somewhat the same, but also not.
The axle beam is the same forging. The 3/4 ton uses 1.75" wide springs and the 1 ton has 2" wide springs, so the U-bolt holes are drilled appropriately in each axle for the springs.
The hubs if single wheel are the same, 3/4 ton did not have dual wheels.
Brakes on the 1 ton are larger than the 3/4 ton, so different drums, backing plates, shoes, and stuff.
Spindles are the same, and I think but not sure on the steering arms.
