"Schrade+" was a trademark, and didn't identify the steel. Nothing unusual there, lots of knifemakers have in-house names for different steels. Queen Cutlery had "Queen steel" for a while, instead of just referring to knives as being stainless.

Prior to the Schrade+ mark, Schrade reportedly used 440C. Originally, blades marked Schrade+ were 440A, heat-treated to around HRC 59. After the switchover (1990-ish, if I remember right), they used 420HC.