Here are a couple of quotes from Jon Broida in reference to Heiji knives from another knife site's forum: - "The only stainless he works with is skd-11, but his feels different from any other one i've ever used. Also, the son studied with Yoshikane, not the father (and though the two work together, its still the father in charge). He doesnt work with vg-10. He has access to lots of steels, but he really only works with the iwasaki special carbon and the skd-11." - "Yoshibumi-san and his son work together on their knives and both are really proud of their work with SKD-11 and the iwasaki carbon. Their SKD really has a different feeling from other makers. In fact, when Yoshibumi-san told me it was SKD-11, i thought he was joking. It feels much more fine grained than any other SKD i've ever used (now from 4-5 other makers). For example, Yoshikane (who's knives i like FWIW) has a very aggressive feel to the edge, generally associated with larger carbide steels like SKD-11. Heiji's feels more like carbon... maybe blue steel. "
Whatever the Heiji semi-stainless steel is, it's just fantastic.
You would be best to ask Jon, but in that same thread you will see them discuss if it's skd11 or skd12. It should also be noted that one of his German distributors lists it as skd12.