You are not required to enter an Importer in your records on firearms that were not Imported.
If you're sure the firearm was Imported and there are no Importer Markings on it, enter "None Marked" in the A&D Record. Do not leave it blank and do not write N/A.
Per ATF Regulation, information entered into your A&D record and subsequent 4473s should only be what is stamped or marked on the firearm.
The information used in the A&D record should match what is listed on the Form 4473 when the gun is sold.