I've found all kinds of counterstamped (letters and numbers) half dollars roll hunting. I have found several Jims Coins and a few one off that were done in the same way as the OP with an electric engraver. I know it is an electric engraver because I used one to mark all of my Snap On tools when I was a mechanic as everyone used the same tools and if you set yours down it was hard to tell yours from the guy next to you even if no malicious intentions occurred when he walked off with your $125 ratchet.