Looking at pics this way and another, this could be a sideplated boxlock indeed, what with the action face being closer to triggers than on most sidelocks, and two screws on the trigger plate. But the lock plates are beautifully executed, it's hard to believe it's not a sidelock!
I had doubts about 16 ga as a pidgeon gun, but then I remembered seeing a Belgian 16 ga side-by-side with the word "Skeet" and images of broken clay birds engraved on the action. It had short barrels choked cylinder or IC, consistent with the routine on early skeet guns, and it looked and felt the part in every way except 16 bore. There might be something about the use of 16s for clay and live bird shooting in Belgium.
