not a US caliber, but my understanding was that the south VN were using 7.62 X 39 at the time, and the NVN were using the 7.63 chambered weapons.
Since my last post, I have done much gooooooooogle searching for 7.63x39 ammo or reloading dies. None to be found. I suspect no such caliber exists. Misprints for 7.62x39 abound. I am a Viet Nam vet. (1st Cavalry Div 11B CIB 1970-71) I heard rumors of ammo interchangeability at the time, and later, but saw no examples of same. Sounds like just another myth about Viet Nam.