Yes, Stinx, a pre ban mag used in a post ban rifle is technically not in compliance with the state firearms laws. The problem is, many local gun shops are telling people that it is ok, but it, in fact is not. I can use a preban high cap mag in my Mass compliant Glocks. I can use a high (re: standard) capacity mag in my Colt Sporter. I cannot legally use any pre ban mags in a rifle or pistol not manufactured before the ban. Fact.
That being said, welcome to the forum, enjoy your stay, and I would be happy to help you with any other questions you may have.
P.S. Many law enforcement officials will not bother you with a pre ban mag, unless you are doing something seriously wrong, are a known offender, or a gang member or street criminal. But I would be safe….