It is easy to flip the 5 and the 7 when talking about these guns, and the Cor-Bon ammo makes me think it is a 357 Supermag, although I didn't know they ever made this ammo either.
If it is a 375 Supermag it is pretty rare and desirable to some. To my knowledge factory ammo was never made, although I think Reed ammo makes some now. Dies are scarce and expensive, and shortening 375 Win brass is a chore.
I don't think the 375 Supermag offers that much more than a 357 Supermag, and bullets are much more expensive. I do like them but sold my 375 Seville and kept my two 357 Supermags and 445 Supermag Dan Wessons.
The prices of Dan Wessons and Supermags seems to going up since they are not being made anymore. I would think a seldom shot 375 Supermag with box and tools would be worth about $750, but a Palmer gun, as noted would require a close inspection.
I wouldn't mind seeing a picture of your gun.
