Buck uses a variety of steels in different models. AFAIK, some are still m-i-C knives. 420HC is their basic steel, but they use S30V and CPM 154, perhaps others as well. Don't know about the slipjoint steels. I would suspect that they are 420HC but couldn't swear to it.
I'd ask this in the Buck subforum. They'll probably be able to tell you pretty quickly.