Somethings don't seem right about the gun. First is the proofs are on the barrels and not on the flats. On the flats it has a name on one side and on the other I'm assuming 'Birmingham'. I've never seen that before.

It is MacNaughton not McNaughton but there are many examples where the engraver misspelt the makers name. As well every MacNaughton I've seen has the street address on the top rib along with the MacNaughton name and Edinburgh.

So there are a few anomalies that raise questions.

Nothing is foolproof to a sufficiently talented fool.