The A suffix was for any firearm, such as the stens too, which would have non-standard parts. Sometimes the parts need major hand fitting, sometimes they are completely different.
I am not so sure there was a major attempt to get rid of A suffixed firearms. I have a few in my collection. I do know the troop trials #4mk1s were specifically listed as to be selected for removal, although many certainly still exist.
One must rememebr that in those days, governments (including ours and Britain) still sold off their surplus/condemned rifles to the public. So just because an "A" rifle was removed from service, didn't mean it was destroyed.