Anyone who knows cannot comment on this due to confidentiality agreements. I am not bound by such things, nor have any commercial interests ... so take my info as you will. I just know that BA is doing more than fare code (which was the original question!) and more than just POS (which is the traditional answer), and the net is that A inventory does not mean you can fly on that flight with an AONEx ticket. How they manage to do this is a trade secret. Part of it is easy to reverse engineer, part is not.

My point is that when your booking falls into this situation, find a different routing or date of travel, rather than beating a dead horse by calling agent after agent to book the A that isn't your A.