I guess I'm just old and therefore skeptical, but my guess is that it is not particularly old.  The Mills name existed, I think, for some time after their golden years and I suppose this one could be an "original" and a "Mills".  But some of the details are not as they would have been in a 'military' belt (edge finishes at top and bottom). If brass is stored - even carefully - it usually develops a patina.  That's why DI's always want the brass polished every day.  There is also a color to older brass that is not in the brass of the last 50 years or so.  Your observations are telling.  If it walks like a duck…...
If it was sold only as an "original Mills", I suppose that is possible, but if you were expecting a military belt, I think it's a miss.  I haven't visited my references on the Mills belts for some time so without doing so, I guess I would just be happy that it's a Mills type belt that holds some proper ammo. This could have been a belt manufactured for hunters and for the retail market. But in any case it's a nice looking belt and should be useable for a long time.