you have a late model bayonet for the m-1 carbine. its probably dated early to mid 60's, because, yes, they did use carbines to some extent in vietnam, mostly used by the south vietnamese.
Special forces and CIA trained many south Vietnamese and Montagnards with M1 carbines throughout the war , A friend across the street from me was refinishing a stock for my M1 carbine , his buddy was their and said he recognized the rifle as one he used to train the natives with in Vietnam , I asked him what years he was in the Army , he replied "I was never in the Army" , he never really said who he was working for , but the only other possibility was the CIA , I asked him if he still had any of his stuff from the war , and he told me he still had his Tiger uniform with in country patches , I tried to get it from him , but he would not let it go LOL , just goes to show you , you never know whats still out there and who your going to meet !!!!
I also tried to get him to do a page on my veterans site , But I do not think he is comfortable telling his story , sorry to get off track , you scored some awesome Bayo's !!!