Check the proof marks again. Mantons don't say J. Manton. They say Joseph Manton. I have a J. Manton and mine has Belgian proof marks. Mine is a cheap knock off and an 1800's replica of a Chinatown Rolex.

I've also seen W. Richards knock offs of a Westley Richards.

I hope yours is the real deal, but please have the integrity of the gun checked by a competent gunsmith before attempting to fire it.

GOD BLESS AMERICA.