You can just call 1-800-731-4000 and do it over the phone.I did it in last September with an old Cooey 22. semi auto rifle that was never registered with no S/N. The only thing that they seemed to care about was that the barrel length was legal, 20" in my case or 470 mm or greater as well as the overall length.When I received my certificate the S/N section was left blank.I believe that the amnesty period has been extended again to May 16, 2011 for non-restricted.You will have no problem as long as you have a valid PAL.I told them that I found it in my fathers closet while cleaning his room after he had passed away, which was the truth in this situation.