Your boyfriend's gun is one of those made for OTASCO, Oklahoma Tire and Supply Co. in 1978. It should also have a special prefix letters of "OK" before the serial number. These guns were ordered by OTASCO each year with a different theme each year. There was a Diamond Jubilee Model 60 also that was made the same year for OTASCO, as they did for other years also.
These guns usually had the less expensive birch wood stocks, but were the same metal as regular 336 Marlins. Prices run the same as the 336 in any bluebook.
Marlin lever actions 1870's-WWI, Ballards, and single shot rifles!