Thanks for the information chaps. The situation that I have is a M1917 bayonet of the Vietnam era manufacture that has no scabbard which I have recently purchased and should be on it's way to me. There doesn't seem to be that many options available for the longer U.S. style plastic scabbards, even reproductions, over here. The one option that I did find was the used original U.S.iconN. MK1 which the dealer says is 17" long but it does have locking lugs, as Jim pointed out. The consensus here seems to be that it won't fit due to the internal dimensions of the scabbard. The locking lugs wouldn't be too much of a problem because they could either be ground off or more likely I would just leave the bayonet sitting slightly proud in the scabbard.

Are there many of these 1960s Vietnam era M1917 bayonets around in the U.S. or elsewhere?

Thanks again for the information.