Nice piece. I see it more as a XF-40; the strike looks pretty complete given the fullness of the parameter stars, but the hair shows a fair amount of wear. Still Charlotte pieces are always in pretty high demand.
As a non-expert, it would seem that it is important to know the typical strike of that particular mint
Are you saying that the rating should be AU 55 or AU 58? My lowering to AU 53 included: on OBS: scant demarcation between nose and field, smudge before face and behind neck. On REV; THE "UNITED STATE" is outstanding, but the "ES OF"
Nice dollar! At first I thought the hair appeared too worn to be AU, but I found this NGC AU55 on eBay (link: http://m.ebay.com/itm/1851-C-G-1-LIBERTY-HEAD-GOLD-DOLLAR-AU55-NGC-/161661615116?nav=SEARCH) with hair that appears weaker than your coin's. This makes me think it's a weak strike rather than wear, so AU is a definite possibility in my eyes.
I'm at AU-53 or 55 but the mark in the center of the neck makes me wish for more images. If I were buying this coin I would need better images or see it in hand. Cool coin. Big money for such a tiny coin.