many states use a firearm's markings and serial number to determine what it was originally manufactured as to determine what it is for legal purposes.
Yeah, Maryland probably defaults to this scheme, but I do know they also have their own separately curated list of regulated firearms with their own designations, sometimes completely at odds with federal law.
I experienced this firsthand awhile ago when I tried to transfer a Mossberg Shockwave into the state.
Federally the Shockwave is just a "firearm", not a shotgun but also not a pistol, because of goofy legal definitions.
Since it wasn't a pistol, I thought I was good to go. Normally in Maryland, you need a Handgun Qualification License to transfer a handgun. (Which is funny, because up until this point, I had already owned legal 80% Glocks. Because you need the HQL to transfer, but not to manufacture, back when manufacturing was still legal here.)
Except I didn't check the regulated firearm list. I'm guessing because of its length, they added it to the regulated firearm list as a handgun. Being on that list, it's not banned, necessarily. But that does mean the state considers it a handgun and therefore you need your HQL to transfer them.
The laws in this state are gay and you really need to dot your Is and cross your Ts when doing any kind of purchase or transfer.
That being said, they're often poorly written and full of loopholes you could drive a truck through, so if you do do your homework, you can usually get what you want.