First the tag if still there is not a VIN tag but a vehicle or car serial number tag. There is a significant legal difference. If present it will be nailed to either the seat riser or floor main sill on the passenger side. The 1 1/2 ton trucks have the plate screwed on the engine side of the firewall passenger side.
If the tag is missing there is no way to verify the serial number from the vehicle. Most states used the engine number as the "official" number on titles pre-WWII. If the engine has not been changed it can verify the title and vehicle match. If not you just have to have faith.
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How Sweet the roar of a Chevy four!