On the cocking piece of a dou 45 Israeli 7.62 Mauser. I've never seen this mark before, and nothing in my references.
The only thing that comes close is a Spanish BP muzzle stuffer shotgun proof. It has the same three circles, but not that little "tail" at the bottom.
It has to be a country's arsenal stamp, like the Israeli Star of David, or the Czech Lion. But what country?
My eyeballs hurt from going over hundreds of stamps, I was hoping someone here would know what it is.