So as a point, assuming a MA LTC-A large cap, if you have pre-ban 30 round mags, and a post ban AR, you CAN'T legally use the old mags in the new rifle?
You're combining two parts of the AWB that really have nothing to do with each other. I believe this is the part you are getting tripped up on:
-The Rifle/Shotgun/Pistol was an "Assault Weapon" or in the case of a magazine holding more than 10 rounds, a "Large Capacity feeding device" manufactured on or before 9/13/94. Note: In order to qualify for this exemption, the device must have existed "in AW configuration" the date the law was enacted. This caveat, of course, is rarely proveable one way or the other.
Essentially they are saying you can have an "Assault Weapon" if it was manufactured prior to 9/13/94 or if it is referring to a large capacity feeding device it also needs to have been manufactured prior to 9/13/94. You can use preban magazines in a pre or post ban rifle.