Try this link for characteristics of Old Chevy Trucks.

It seems like vent windows in the doors would be the key. The '51 had them but the '49 didn't.

They sometimes used engine numbers for the VIN's which could get confusing if someone swapped engines.


1948 3/4-Ton 5-Window Flatbed Chevrolet

34 Years. Now with a '61 261, 848 head, Rochester Monojet carb, SM420 4-speed, 4.10 rear, dual reservoir MC, Bendix up front, 235/85R16 tires, 12-volt w/alternator, electric wipers and a modern radio in the glove box.