The date of manufacture makes it a "relic". Its uniqueness or some other feature makes it a "curio".
The "relic" thing happens automatically when a firearm hits 50 years. The "curio" thing happens if a request is made of the ATF to make that determination. The request doesn't have to come from the importer - anybody can make it. If you happen to see the list of C&R firearms, there are a lot of individual guns on it, identified by serial number. A lot of them are short-barreled Winchester rifles. Individuals have requested them to be declared curios based upon their historical importance.
Well we don't rent pigs and I figure it's better to say it right out front because a man that does like to rent pigs is... he's hard to stop - Gus McCrae