It looks very much like a number of Italian hammerguns I've seen, including Bernardelli and Beretta marked ones. The sidelock plate has that extension in the front into which the screw is placed, which you see on a number of Italian hammerguns from that time period. There was a Beretta on one of the for sale sites for a while which looked almost identical, except for the engraving. Might still be up if you do a search.