I admit the old memory isn't what it was, but that doesn't look like the gun I had. I recall a tang safety, and don't recall any "mystery part". Nor is anything like that shown in the parts in the GPC catalog, though it does show the trigger guard safety. (They list it as "old model".)
As I said, my gun had no recoil slide lock, and that one appears to be an "add on" improvement, though a clever and effective one.
In other words, just more confusion in the world of Savage-Stevens-Springfield.
Perhaps this is not right, but how about trying www.jouster.com
which has a forum devoted to military shotguns. They would probably be better able to help.
BTW, great pictures!