Regarding Maryland State gun registration, things have gotten easier.... thank goodness.
The MSP and the ATF have conversed and agreed that all serial number markings will be as per the ATF guidelines.
For "Lowers" it will be the manufacturer's name (which would be yours). The manufacturer's city and state. My abbreviated FFL number followed by a hyphen and three digits. Then the caliber. A caliber listing of "MULTIPLE" is acceptable since barrels can be readily changed.
For Polymer Pistol Frames the serial number is my abbreviated FFL number followed by a hyphen and three digits.