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.
"If there are no dogs in Heaven, then when I die I want to go where they went." - Will Rogers 1879-1935
RIP Riley, Molly, and Murphy... well done lads! AMOR AETERNUS
ISEK... and freedom of speech #12