Chris, Without detailed pictures we cannot give you even a ballpark value. My suggestion would be to go to an auction site such as Gunbroker and search Completed Auctions for a similar gun. Be sure and look at those guns that were sold--- not the ones that relisted. On a side note most likely your grandfather was mislead when he told you that it was "designed for 7-1/2 shot". It's likely that he read that information off the hangtag-- 7-1/2 shot was used for pattern testing.