It's a very lustrous AU55-58. Look at the cap and the hair smoothness adjacent to and below the cap. That reverse I thought had wear, but these pictures don't show it, or at least not what I thought I was seeing. They'll have to do some major rationalization on that cap and hair smoothness to call this MS, though, in my opinion.
Or it could be due to a weak strike.... The 1921 Peace dollar is infamous for its awful strike. Look at the luster in the 3rd picture of post #12 - it is complete across the flat area. The key to determining if missing detail is from a weak strike or from wear is to look at how the luster behaves in that area. That means this coin is weakly struck, but not worn - and thus deserves an UNC grade.
Thanks for adding more pictures. Definitely weak strike with a lot of chatter in the devices/hair. The "scratch" shows up on your one of the new pictures, but not the other. If you send it for grading, please let us know the results. Peace Dollars are one of my favorites and the '21 is hard to find fully struck.
I think if you send it NGC it will be called "AU DETAILS" - I'd keep it raw and enjoy it. Very beautiful coin OP!
The "flat area" you appear to be talking about is that large, lustrous area covering the ear. I'm talking about the missing detail and luster in the cap and hair proximate to the cap. In all fairness, I can see some faint streaks of luster in that cap area, but much of the detail is smoothed down. That's the highest area, though, or "deepest" on the die, so if any area isn't goin to take all the strike, it is that area, I guess.
I thought AU on the first pictures but I think it would slab an MS61 or 62 on the second pictures. Put a third set of pictures up. Maybe it will go higher! It is very hard to grade a coin from pictures unless they are very good. And yes, typical lousy strike.
Typical mushy strike due to lower striking pressure. The high relief dies were breaking after just a few strikes which proved to be very costly. The dies lasted a little longer under lower pressure, but were still unacceptable in high relief. A re-design in lower relief was necessary so in 1922 the relief was lower and there were fewer striking problems.
In the old days this is XF-40. Personally I am only between XF 45 and Au 53. I like the 1921 but I wouldn't buy this at MS price.
I don't see the wear that I saw in the first photos. That said, the coin looks to have been dipped. The scratch on the face is troublesome, and although I love these coins, I'd pass on this one.