as from what i can tell of the new law, you can own, use, transport and store said magazine(s) if you purchased them BEFORE the new law went into effect. If ya don't save receipts, then the burden of proof is on the owner being questioned. border states to those who have a mag capacity ordinances are under extra scrutiny and any FFL who knowingly sold the "illegal" device may receive extra scrutiny for any and all transactions thereof.
STandard capacity magazine possession is legal.
Now: the sale, transfer, or whatever change of ownership is illegal in MD. An out of state transaction is legal.