This has been interesting. After putting all of these responses together, I've come up with the following theory: "31", "sbox","70", and "170" refer to the 3100 truck. The 70 and 170 are shorthand for the frame length of 173 1/8 inches, which would therefore be the 1/2 ton PU with the short bed. I believe the "31" under the hood indicated which emblem went on the side under the Chevrolet emblem. My hood, which I don't know the origin of, had "82" on it. It might stand for a model 3802. So I put a "14" on mine, to stand for 3104, the 1/2 ton PU. But I could also have used a "31" there. Seems that different plants used different shorthand or maybe even different crews made up their own.
Question for afu45: I viewed your photo. Your "170" had to be written there before the cab was joined to the chassis. Once the cab was mounted, the number would be covered by the engine. When it came time to mount the bed, they couldn't have seen it, right? So what was the point of writing it there? I must be missing something.
