The shops are wrong, my wife is an attorney for the Ma Superior Court, and we have looked this up before. When I get more time, I will try to find the actual written law and link it to you. The thing is that most people, including most law enforcement, do not have a clear idea because of the way our laws are written in this state. I am only sure because of a trial decision she had wrote for a case she worked on.
This law is often confused by most leo's and gun shops, and they tell people that all it has to be is a preban mag, but if you get caught by someone who knows the law, and has been having a bad day, it can really ruin yours. Just a heads up. Many leo's will not even worry about it unless you are doing something terribly wrong otherwise.