Beveldown, you've got a very nice BPE double rifle there! My own is in much worse shape, but a good shooter and reliable killer too. Mine is in .500-.450 #1 BPE and is signed by A. Robertson & Son, Wick (Scotland). It also has conventional rifling, six square grooves wider than the lands, rh twist. Steel barrels, C.Homer's 1877 design foreend latch, pre-1887 Birmingham proof marks. Otherwise it so much resembles your McNaughton, I suspect both rifles originated in the same Birmingham shop, made for retail by the respective Scottish gunmakers McNaughton and Robertson.