Years ago I had an opportunity to stop at the Ruana Shop, after Rudy passed, and at that time I had an old knife marked 12B sticker,and asked about the Studebaker leaf springs. Apparently it was the carbon content in the steel that Rudy liked. Those springs were becoming harder to find in the Missoula area, so Rudy looked elsewhere. A "medium" temper was the thought behind the M stamp. More rare are an S stamp, for salt water temper, and a T stamp for a "razor" temper.

Pap