Joe,
It's a very cool time piece. For those that do not know this, several suppliers sold fiberglass versions of OEM fuel tanks for the drag racing crowd running in the early Pro Stock class back in the 70s. Rules then required many of the factory defining features to remain visually identical to the factory parts, so the bikes did not losee brand identity on the track.
Fiberglass front and rear fenders, seat pan, and of course fuel tanks were made to look like the originals, yet be lighter for racing. I'm not sure about a value though... it's certainly not going to be interesting to the street bike crowd, since ethanol laced fuel will break down the resin in short order. But for a dedicated racer who wants to keep the factory k model look and run proper race gas, very cool. I'd be curious how much lighter that tank is than stock. I have a vintage Dunstall glass tank that is just BARELY lighter than the OEM steel tank. If you have a scale to weight the glass tank, I'll weigh a factory steel k model tank.
George