Johnny,
On most GM factory installed seat belts after the year of 1966 the year of the belt was on the belt. A label was sewn at the attachment end with the manfactures name, year, and model. Example: Robbins (manfacturer) 1966 (year) model 300 .This belt would be for a 1966 Nova station wagon. Seat belts sold as accessories and dealer installed were identified by a six digit part number (example:986366-1965 front seat belt-saddle-all exc. trk. & vette.) To my knowledge this part no. is not on the acc. belts.
Dandyd