I've collected, scanned, cropped and edited for clarity, and noted the printing date on the various GMC wiring diagrams.
They are pretty large clear images and should print well.

They can all be found in this album
https://goo.gl/photos/H26qPfBHb7eS15698

Here is the 1951 GMC diagram from the owners manual.
https://goo.gl/photos/UvtdEAKaUC6EATES9


To answer the original question:
The 1951 conventional truck wiring diagram is the same info as on the 1953 diagram. The 1953 diagram however includes more info for the optional hydromatic, optional electric wipers, and optional electric two speed axle.

The 1951 and 1953 COE wiring diagrams I believe are exactly the same, no changes/additions.

The 1949 diagram is drawn in a different configuration and hard to compare, though it obviously functions about the same I suspect there might be a few connection details/differences. The wire color code is a legend on a separate page. I believe there are some differences in the color code for the same wire function between the 1949 and 1953 diagrams.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
---All pictures---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-