Parabola, I have an almost identical single .410 that you pictured. Mine is by Ebrall Brothers of Shropshire and was bored out from a .360 Rook Rifle. Possibly yours is too. If you find in the Proof Marks a number something like 40 or another two digit number that would be the bore of the Rook Rifle. If it's got re-proof marks then that may be how it started out. Lagpus.....