I don't believe that anyone has touched the part of your questions regarding Massachusetts Law. Specifically regarding High Capacity Magazines.
In order to possess a high capacity magazine, which is a magazine that holds more than 10 pistol rounds, 10 rifle rounds, or 5 shotgun rounds, it must have been manufactured before 9/13/1994. Your class-A LTC gives you the ability to possess these magazines and conceal carry legally.
The date you are referring to (1999) corresponds to the Pistol Compliance List which is basically as follows. In order to purchase a Glock, or a firearm that does not fall onto the Massachusett's Compliance list (or the AG's "list" which is rumored to exist) is must have been IN the state of Massachusetts and manufactured before 10/21/98. This only applies to Dealer transactions and does not apply to people who are moving to Massachusetts or private sales.
Also, in Massachusetts it is possible to own a firearm capable of firing without the magazine inserted into it. The M&P series is an example of one such firearm. If you look, yours should have a warning right on the slide that states it is capable of firing without a magazine inserted.
Here's a link to a forum in which I originally learned the laws of our cruel state.
Massachusetts Laws