I know of an H&H 8 bore "smooth bore paradox" (?) for 6KGBP with the same case length and charge and barrel length as this one. I don't know if it has a rear sight as this one does though
Look closely at the locks on this one - looks like they are for a straight gripped gun. Pistol grip H&H back action hammer guns generally have a bit of downward curve to them. These look straight - especially on the lefthand view.
For this to have survived uncased with virtually no loss of colors - especially on the underside - would be truly incredible.