I.M.O. I don't think it is a proofmark as the proofs I have seen are on the chamber of a firearm, on the area of the barrel closest to the receiver. Then you have firearms like a Bearcat for that have proofs on the barrel and the cylinder.
But I can't recall ever seeing a proof on a receiver.
Chet15
nope, that is not a "proof mark" from any foreign country, most likely its one of the guns from the "International Exposition Center located in Cleveland ,Ohio ( the I-X Center) they were used to kill critters and vermin on the off hours out back....... :roll:
Once the city took over the property, they had to get rid of any and all firearms 8)