Directly from the ATF web site....
Caliber or Gauge Name of Importer City & State of the Importer.
The importer must ensure that these marks of identification appear on each firearm. If the manufacturer did not mark them, the importer must place the required markings on the firearm within 15 days after their release from Customs and Border Protection custody. The Director, Bureau of Alcohol, Tobacco, Firearms and Explosives may authorize other means of firearms identification upon receipt of a letter application from the importer, submitted in duplicate, showing that such other means of identification is reasonable and will not hinder the effective administration of the regulations.
Also in my experience a lot of English guns may not have a serial number or just a serial number on the barrel loop. You would be required to create a number and have it stamped or laser engraved on the frame/receiver.