I am sorry for the delay. I was hoping to find an AR556 at my local store but I have not.
I did look at my Stag Model 8 AR. I has maybe 1000 rounds through it. I has the black finish warn off it like your gun has. Again, I firmly think this is totally normal.
I also looked at my wife's Stag 1L (Left hand). It has not been fired very much at all. It still has all of it's black finish in side.
But I think the thing you are questioning is the small carved out section that is very clear in your pictures. My gun, the Stag 8, has such a place too. But it is not so big as yours.
Perhaps it is a matter of the photographs. Perhaps you should make sure that your pin is moving easily and you have enough lubrication. ARs are known to like more lubrication than normal guns.
I'm afraid that's all I can offer you at this time. However, if you can think of something I'll be glad to do what I can for you.
Regards,
Spike