Schrade "plus"
It's actually not a single type of steel.
I've seen that stamp on at least three different kinds of steel, and one of those was carbon steel, not stainless.
I'll see if I can find a reference, but one was 420HC (stainless), and the carbon steel was a 4-digit number starting with 10 (10xx).
Think of it as more of a "branding" thing, as opposed to a "type" thing.