Most interesting find indeed. Not much help here, but a friend of mine has a 6 bore double ball and shot gun, that was re-furbished by H&H and bored to 5 bore to bring some pits under control. The bores are now as if new as-is the rest of the gun. I do need 'Wil' to take some pictures and send them to me.
The re-boring might have been what happened to this one, although it's obvious it wasn't refurbished any time recently?