Hey there, and welcome to the Yard. Through hardened SR101 is, to my understanding, 52100 steel with the proprietary Busse heat treat. What makes it "through" hardened is that the entire blade is the same hardness. Since SR101 is not quite as tough as SR77 or INFI, larger SR101 blades from the Swamp were differentially hardened. That is, about the first three quarters of an inch away from the edge is full hardness, while from that point to the spine is softer. I hope this makes sense. If not, hopefully someone else can explain it better!