What firearms are considered to be curio and relic firearms? | Bureau of Alcohol, Tobacco, Firearms and Explosives

this has been a part of C+R regulations for as long as i can remember -- military imports with replaced sights , grips , non original added safeties , + etc. -

[ ATF has recognized only complete, assembled firearms as curios or relics. ATF’s classification of surplus military firearms as curios or relics has extended only to those firearms in their original military configuration. Frames or receivers of curios or relics are not generally recognized as curios or relics ]