It should be 425 steel. The 425 may have an M behind it (425M) which I think means modified. If I am wrong, someone will corredct me.
From the begining, Buck used 440C until some time in the 80's. In 1994, they switched the steel to 420HC. All of these steels are great, but I like the old 440C best. It really comes into its own when the edge has been thinned out like the edge2000 knives. I have a 2 dot 112 (w/ 440C steel) that has been thinned down to about 12 degress per side. That 112 slices all day and holds a great edge.
Go ahead use it. These knives are too good to just sit in their sheaths.
John