The answer is in the Instruction Sheet For ATF Form 6NIA (5330.3D).
23. Only nonimmigrant aliens seeking to temporarily import firearms from Canada should answer question 12. This question requires all nonimmigrant aliens seeking to import firearms from Canada to provide their Firearms Possession License Number and/or Firearms Possession and Acquisition License Number.
In addition, only nonimmigrant aliens seeking to import firearms from Canada should answer question 15h, listing the Firearms Registration Certificate Number of each firearm they seek to temporarily import.
If you do not have a Firearms Registration Certificate Number for a particular firearm, you should write “N/A”.