Were Mini 14's regulated in Maryland? That's the only way they could prove when you owned it, as long as the rifle is old enough to be pre-2013. Or am I missing something?No.
Unfortunately, you can't because the rifle would have to have been registered to you prior to 2013 FSA.
It matters not how old the rifle is. It matters when the rifle was registered to a specific person in Maryland which had to have been prior to the 2013 FSA.
I'm not a lawyer.
Now you "might" be able to keep the stock out of state. Put it on, in a legal state, and shoot it there. Then convert it back to unbanned status to return to Maryland.
But once again, I'm definitely not a lawyer and not giving lawyerly advice. Just a layman's opinion.