This reads like a grandfathered law. IDK MD laws very well though.
I assume there's a date that this went into law, so obviously any non-standard magazines manufactured after that date would be illegal. Not sure how they could enforce any made before that time unless they can literally catch you buying or selling one.
Not grandfathered at all, possession has never been illegal, even back when the limit in Maryland was 20. No manufacturer date required on any magazine to own it/use it in maryland.