I'm not quite sure I'd agree with that 'A' explanation Brad. If that was the case, then something as important as this, especially for hard pressed Armourers, would have been mentioned early on. As it was, the announcement (regarding A suffix rifles) first appeared in SAI's..., small arms instructions and was then carried over to the EMER's.
You could usually spot why an A suffix rifle was marked. The most common fault that I saw was oversize sear axis pin holes, ring punched to 'correct' it. Being a common one, it always featured in trade tests of course. Another was barrel thread indexing meaning that while the factory could usually breech up and index the barrels with the thousands they had, down at lowly Field workshops, with about 20 or so in the breeching-up bay, we were pretty well stuffed.