I would not buy any aftermarket magazines based on the seller's representation that they were "pre ban". I made that mistake once with AR 15 magazines, only to find out they were new manufacture. They were shipped out of state for storage until I can figure out what to do with them. But, I digress.
To the best of my, and that ofseveral people who study the laws in MA, no one has ever been charged for mere possession of "high capacity feeding devices." Those other people are in court on a lot of gun cases and are very conversant in the law.
Everyone charged under that law is also charged with serious, often violent, felonies. The police and DA will throw every conceivable charge at a person, knowing that most of them will be bargained away. Which is why only one person has ever been convicted on the 40+ year old "Bartley-Fox" law for mere possession of a long gun without a Firearms ID card.
For example, if a person were to be arrested for drug possession with intent to sell, and they had a firearm on them, they'd be charged with the original charge, possession of a firearm without a license, possession of ammunition without a license, and if it was a so called "high capacity" magazine, they'd be charged with possession of a high capacity feeding device without a license. Pre or post ban would really matter.
Now, if the gun happened to be under the seat of the car they were in, that would be another charge for improper storage of a firearm.
Most of those charges would be tossed in the inevitable plea bargain (Remember, in the halls of justice, the only real justice is in the halls!).
All of which is to say that if I were to be in a position where I was going to run afoul of the law, pre versus post ban magazines wouldn't even be on the list.
Which is why I'm not worried about any of those 69xx magazines, and worry even less about the 2nd Gen magazines. Which, while curiosities in about 45 of the US states, are actually pretty valuable here in the PRM.
It's not an issue at all with M&P semi autos since they were all introduced after the ban and so anything with more than 10 rounds is by definition prohibited.
Glocks are a bigger issue because they were initially produced before the ban, but have continued more or less the same since the ban. People really obsess about pre and post ban Glock magazines. It's been several years since I had a Glock 23, so I forgot what magazines it came with. It was a refurbished LE gun, thus salable in MA, even though dealers can't sell new ones here.
Anyway, I'm not worried about either the 69xx or 59xx magazines I have.
ETA: Here are the aftermarket ones I'm most worried about getting hold of by mistake.
The problem is that they look very much like S&W 6900-series pre-bans but they are post-ban and could land me in prison for up to 10 years... plus get my license voided and collection confiscated just for good measure, of course.
Shipping looks like $8.00 per magazine or pair of magazines.
Absolutely there are aftermarket magazines out there. 
Sorry GaryS, but I have to do it...
