Delrin first became commercially available in 1960, IIRC, and started getting used by cutlers as a handle material almost immediately as a low cost replacement for bone and stag.
Western started using it in 1961, replacing all stag and bone usage. I think I read somewhere that Case started using it in late 1960 or early 1961 as well. No clue as to when other companies started using it, but late 1960 is the earliest it could have been used. And your knife is an example that Kabar also was using it prior to 1965, at the latest. I'd bet they also started using it in 1961.
Obviously, there were other types of plastic used as handle material prior to 1961, but Delrin just kinda jumps out at you, almost always used as a faux stag/bone replacement.