Very strange. If this is a Belgian rifle, there must have been proof marks on there somewhere at one time. The 374 will just be a number from a batch, and the others have me baffled.
The Y is a Y, and if the thing in a circle has the same orientation, then it could be a monogram: W inside G.
I can only find one Belgian gunmaker's name that fits - William Grah. Who did, in fact, make sporting Martinis.
But where oh where are the proofmarks???