All handguns were required to have serial numbers by Federal law after the 1936 National Firearms Act.
Shotguns and small bore rifles were not required to have serial numbers until the 1968 Gun Control Act.
Prior to the 1936 NFA, higher priced & higher quality guns were alway numbered, such as those from Colt, S&W, Winchester, Marlin, and others.
Less expensive models from Winchester & and others were not.
Mossberg didn't quite see it that way, or sell for as much.
So most of them weren't numbered until after the 1968 GCA became law and required all firearms to include a serial number, manufactures info, and importers name if it applied..
rc