We are going to need to know more about that gun before jumping to conclusions.

I never like to say never, but the Tolley boys would never have made a SxS double punt gun with sight-leaves to 150 yds! They would (almost) never have made a shotgun with a capped pistol grip.

Also, Tolley made a shed-load of plain-finished fowlers, and would have been extremely unlikely to have put that standard of engraving on a punt-gun!

IMHO this was a big double rifle that has been bored out to comply with a shotgun certificate, when the laws tightened up in the UK.

...but without an examination of the proofs, who knows?