The actual law (MGL Chapter 140 Section 131M) says, “No person shall sell, offer for sale, transfer or possess an assault weapon or a large capacity feeding device that was not otherwise lawfully possessed on September 13, 1994.” The thing about that is it doesn’t say that you had to be in possession of it at the time, meaning as long as it was lawfully possessed by someone before then it’s kosher. As you can probably imagine, that’s almost impossible to verify, hence why the guidance of mags made before that date being legal. If they were made after, there is no way they could have been lawfully possessed by anyone since they didn’t exist yet.