Firearms automatically attain curio or relic (C&R) status when they are 50 years old. Any firearm that is at least 50 years old, and in its original configuration, would qualify as a C&R firearm. It is not necessary for such firearms to be listed in ATF’s C&R list. However, if your C&R item is regulated under the National Firearms Act (NFA) and you desire removal from the provisions of the NFA, you must submit the firearm to the Firearms Technology Branch for evaluation and a formal classification.
Start reading here:
http://www.atf.gov/firearms/curios/2001index.htm" onclick="window.open(this.href);return false;
Pay attention to Sections III, IIIA, and IV.
If I understand the process, the s/n's you see listed are those where a variance was granted, but you should be able to tell if yours would qualify.