I'm not aware of anyone getting jacked up just for mags. It's usually an add-on charge and comparatively insignificant. I would love to see what a jury would think about "well if the little sides at the top of the magazine are angled slightly outward, it's evil and illegal and babies will die, but if the edges are a little more upright then we're all safe and fluffy". It's nonsense.
a.) I believe there have been posts here from (perhaps now banned) lawyer members mentioning cases with just mag charges. Rare, for sure, but I think they have happened.
b.) I would NOT want a Massachusetts-based jury thinking about the mags. Maybe I'm not giving the general populace enough credit, but a line of:
Prosecutor: "The law says no more than 10 rounds"
Defense: "Only if it was not possessed before 9/13/94"
Prosecutor: "Prove it was pre-94"
Defense: "The burden is on you, but take our word for it"
I don't have a ton of confidence that you wouldn't get some folks who would be willing to side with the prosecutor without clear evidence on its status (date stamp, out of business manufacturer, etc.).
In all, you probably have bigger things to worry about. I would rather take the time for find magazines that can easily, with very little uncertainty, be said to be preban. Obviously, depending on the manufacturer, this may not always be possible.
Don't get me wrong, I'd love to be able to say "hey, they have to prove it, innocent until proven otherwise!" The cynic/realist in me can't always be that positive. In a fateful scenario I hope no one encounters, I'd prefer to be able to say to a LEO, "this company went out of business before 1994" or "the date of manufacture is stamped here, 1992" etc. Whether or not the LEO would accept it is another matter, but I'd rather be able to start with that from the get go.
Mokdad's case was probably the best precedent on mags in MA that I can remember, but if I recall it correctly, the magazine issue was basically tossed because there was no testimony that no one else possessed them pre-94, not that the prosecution couldn't prove they were pre-94, so kind of a technicality. Actually, it's a little scary--what kind of testimony could someone offer for an unstamped/unmarked AK mag?