Sorry...Ruger never made a .30 Carbine in stainless.
If it appeared in these RF classifieds, somebody either nickled it to look like stainless or they polished it out.
It is kind of interesting that R. L. Wilson put such a gaffe in his book, because he copied most of the other technical information from other authors. No other author that I am aware of ever made mention of a KBN-31.
Chet15