Does it have a butt plate with image of a waterfowl and intitials S.J.D. & Co.?
If so, made by Simonis, Janssen & Dumoulin of Liege, Belgium for Meacham Arms Co. of St. Louis, and probably obtained by your grandfather after immigration.
Later: Rereading your post, the "rear wood plate" you mention probably replaced the hard rubber original buttplate I cited. But if you remove the barrels, they will probably have several Belgian proofmarks on underside, such as those on the "Union Machine Co" example shown here (scroll down past the revolver and rifle):