I find, among the dozens of Mortimers in the book, The Mortimer Gunmakers 1753-1923, only a Robert Mortimer. The book does not associate him in the period you say the gun is from, but rather he was writing a family history as late as 1930. Let us see a picture of the gun and proofs. If you cannot post pictures, I would be glad to do it for you. darylh@theglobal.net