Those must be the Chevy numbers.
The GMC numbers from my 1939-1952 light duty parts book 1/2 ton to 350 or 370 series.

Group 10.0290 Skirt - Front Fender

...Nevermind, I can't make sense of it and don't want to transcribe the whole section... see attached picture of the book.
Attachments


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
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-