OK, something is very strange with my AH install. I had downloaded only my "The Deacon" P51b skin, and the game wouldn't show it in the hanger. However, for all other planes, where I downloaded all skins available for that plane, I am able to select any skin in the hanger for that plane. I didn't realize this, as I hadn't tried it (only flying my P51b so far).
I will now download all the P51b skins, and will see if having more than one optional skin in the folder makes a difference. As you suggest, it may also be a minor bug.
TDeacon