I don't understand.the barrel still turned with unscrewing.
Funny, I don't see a pin close enough to the barrel to have anything to do with it. Rather, there's a lever near the pin which is probably retained by said pin.
Sure enough there's a pin. No, wait, it's a spot on my monitor. Your eyes are better than mine because I see only the one pin which is too low on the frame to prevent the barrel from turning.Your looking too low. On the flat section about 3/4 down I'm seeing a pin ~ 1/8" which is in the correct position to secure the barrel.
I think he is referring to the spot indicated by the red arrow
.View attachment 909535
I believe you are correct. Looks more like a plated-over flaw in the frame. If by chance (or NO chance) it is a pin it would need to be a through pin, and visible on the other side also.