HKFan9 I have a few friends with them, two of which are police. You are not attaching the stock to the frame of the handgun... you are not altering the frame to accept the stock in any way. That frame is registered as a handgun, NOT a rifle. Like their website states, it is just an accessory. Think of a buying a .22 conversion kit for a 1911 and such. You are attaching a slide that has a longer barrel and stock, but the FRAME (IE the registered part) is still a pistol frame and registered as one. I purchased one used off a state trooper friend, then traded it back into a shop. I have watched them take 1911's and Glocks and field strip them to install the upper before a carbine match. I don't see the issue. Like others have stated... they have been around forever.
It doesn't matter whether the stock attaches to the frame, the slide, the magazine, the grips or any other part..............you attach a shoulder stock to a handgun you have manufactured a short barrelled rifle. That makes the handgun an NFA item subject to a tax stamp.
I have no idea what internet lawyer told some of you that the frame must be altered. Once a handgun has had a shoulder stock attached in any way it is an NFA item whether the stock remains attached or not. Without the stock attached it becomes a "firearm made from a rifle" meaning it remains an NFA firearm.
Being around forever is immaterial. The manufacturers of the MechTech are not making an NFA item.....you are. They dance happily to the bank while you ignorantly violate Federal law.