This is WW Greener’s Self Acting Striker, patented in 1868. It was his first patent after he quit working for his father and went out on his own. At the time primer quality varied quite a bit and strikers often penetrated the primer and stuck in them. Greener’s solution was to manually force the striker to retract before opening. You can see a small protrusion on the throat of the hammer engaging a stub on the striker. As the hammer was moved to the half cock position it pulled the striker away from the primer. Obviously this lock does not have the rebounding feature Stanton had patented about the same time. Very few of these were made. I love the graceful sweep of the hammers and the way they lay in full cock. They remind me of a racing horse in the starting gate eager for a race to begin.
Hammers certainly gave a craftsman a palette for artistic expression.