In NJ we cannot own any magazines over 15 rounds, pre-ban or not. Our state AWB wording was literally plagiarized by Biden and friends when they wrote the text for what became the 1994 AWB.
NJ firearms laws are written in vague terms. Some people buy 20 round AK and AR magazines that have been permanently altered (pinned and welded) so they can only hold 15 rounds. These magazines are legal in the sense that no one has been prosecuted for possessing them (yet).