You could send that photo to Mark Silver, a fine gunmaker and friend of Monte's - he has a nice website. He would be able to help you, I expect. Don't know how to get hold of Mandarino himself. But I would bet that is a Mandarino rifle.

John Bivins said once that Monte was the greatest gunmaker of the last 400 years. When you look at his whole output including the breathtaking flintlocks he's made, you start to believe it's true.