The curio and relic designation for firearms States that anything over 50 years old is considered a curio or collectible. That means that anything manufactured before 1968 is considered a curio or relic. The problem is that just like many firearm rules A lot of dealers and sellors are not aware of the rules and will not as they see it take a Chance and ship a gun of that age as a C&R.
Sometimes, if you explain to the dealer that the gun you're looking at is a C&R item they will agree to sell it as such. However, don't expect many of them to do it on their own.
A fire arm that is over 100 years old is considered an antique and can be sold with no paperwork of any kind