I know of 5 of these (of which I have seen 4 close to) - and they all differ somewhat. I suspect that there was never really a finalised design.
As to the stories that they were difficult to set up - mine wasn't, and the other owners I know and have talked to have not had any trouble either.
My own guess - and its only that - is that it was more expensive to make, and had little or no real advantage, so was phased out. Lets face it - there is VERY little wrong with the 'A' frame design - the Advance and Retard is pretty crude - but in this respect the double A is really no better, and the A frame does need to be correctly set up - in this respect I think the double A is as good or better with its separate crutch.
My architectural, and my double A are both capable of accuracy of a second a week or so, so nothing to choose really on timekeeping.