Hello Mike,

I've been through all my Sharps Borchardt Literature and photographs but I have been unable to find any account of such marking, or even mention of it.

Whist some Borchardts were sold to State Militias, I cannot see it being an acceptance or inspectors stamp as it is hidden from view.

Seeing as each action had to go through many machining processes it's hardly likely to be a machinists mark as a dozen or two different workers would be involved on this action during manufacture.

You'll just have to wait and hope someone else comes up with a possible or plausible explanation.

Harry