A little late to the dance but I can add a bit more to the story.
Northern Bob is correct.It was a patent by Henry Parsons and an image of a stone masons hammer was stamped on the barrels.I did own a hammer gun with this patent which surfaced locally about 40 years ago and has since found a home I believe in Eastern Canada.I have seen about 3 or 4 examples of this patent over the years.One was on an early hammer double rifle by Thomas Horsley. I will attach below a pic of the reference to this patent in Crudgington and Baker Vol.2 Page 212 and a pic of the lump and patent stamp on the hammergun I owned.Hopefully the line down the center of the lump is visible.
