Is there a VIN on the frame? Is it an aftermarket galvanized frame? It’s possibly it was rebuilt from a 1970 series and the seller is just a moron who doesn’t know anything.
Any problem getting more info on its origins or a bill of sale for a 1970? It’s not like a 1985 is really going to make much of a difference in value to you. You’ll still likely need an agreed upon value from insurance regardless of the year.