You are all wrong and you are all right. The ATFE has a list of "approved curio and relics" You can find it here:
http://www.atf.gov/publications/down...-p-5300-11.pdf .
As a general rule a gun that is 50 years old (by manufacture date) is elligible. Just remember that the burden of proof is on YOU the purchaser and the seller. If ther are no records of manufacture dates Vs. serial number or no serial number, you are out of luck. Now if there is an obvious reason that the gun must be more than fifty years old, i.e. the company went out of business (i.e. Eastern Arms Co.) or discontinued manufacture of that model more than 50 years ago (i.e Wichester model 1897) you are good to go. Short barreled guns and machine guns still fall under the old rules.