Generally firearms that are 50 years old or older, or of particular historic interest, are recognized by BATF as eligible for direct purchase by C&R license holders. Firearms in current production are usually not - despite age.
For example, the CZ82 pistol in 9x18 is less than 30yrs old but was the military sidearm of Czechoslovakia during the cold war period.
On 1 January 1993, Czechoslovakia ceased to exist - splitting into the Czech Republic and Slovakia. Thus, the CZ82 is of historic interest to military collectors and was declared C&R eligible. The handgun is still commercially produced as the CZ83 in .380 which is not C&R eligible.
It does sometimes get confusing. As I understand the process, firearms like the CZ82 have to be brought to BATF's attention for review. And then await their decision.