I can't imagine that a presecutor would do hours worth of research to try to determine if an AK, AR or any other magazine were pre ban assuming it was not blatantly obvious.
I can't imagine a prosecutor would do that either. They would simply take the easy route - put the burden of proof on you. As I am not aware of any case law in this regard, you would need a damned good lawyer to put that burden back on the DA and establish said case law, and there's a very real possibility that you might spend some time in the klink and lose everything you own in the process. Given what just happened to that poor SOB in Manchester-by-the-Sea, ANYTHING can happen in Massachusetts and much of it could be very, very bad for otherwise law-abiding gun owners. As Woody Allen said, there's a word for people who are convinced that the world is out to get them - perceptive.