FarNorth - Sorry to take a bit to get back to your post. I followed up on Kevin's idea that the "VIN" 5AN018297 does possibly seem to be an engine number, so I dug back into the GM Heritage data file to see what may be found to try and clear that up.
The 1941 Chevrolet Truck data PDF shows more detail in various places, some that seem to be difficult to follow/understand. In the 1st page of "Basic Model Information" (pic 1 below) it indicates that a One ton truck (Vehicle Type) identified as Model Name: One Ton (RPO) with a 134-1/2" wheelbase chassis would have been identified with the Serial Letters: YR. The Serial Letters "AN" are shown to apply to a 3/4 ton Panel truck on a 134-1/2" wheelbase chassis that is also further identified by the designation "TSL".
Could it be that your "AN" Serial Letter sequence data plate was originally on a Panel Truck??? That Panel would have used the same long 134-1/2 inch wheelbase chassis as the 1 ton pickup. That may account for your mention above of the truck having a possibly non-original "VIN plate".
If you look further in the GM Heritage document, in the section "Unit Serial Numbers" (see pic 2 below), that information would have your "VIN" decode as:
5 = Kansas City Plant
AN* = 1941 3/4 TSL (3/4 Ton Special Panel with 134-1/2" wheelbase) *year/model taken from the first pictured "Serial Letters" info
01 = January (build month)
8297 - 7297th truck built at the Kansas City Plant for model year 1941
Anyhow, have you measured the wheel base to confirm it's 134-1/2"??? Feel free to continue to ask questions as you like and we'll do our best to help out.
