Some states register with the engine number (which on some cars was also stamped on the frame). The OP's "VIN" may be one assigned by his state's registration agency.
This was before the Vehicle Identification Number became standardized as a 17 character alpha/numeric string with each position having a specific meaning, allowing you to decode a modern VIN, the first character being the country of manufacture and the second, the company. Other stuff like body style, engine, transmission, etc are also included.
