Herodotus says what the letter the names ended was, it's sigma (Σ). In Dorian alphabets sigma was called san. Originally san was a different letter than sigma in Archaic Dorian scripts, Μ. This letter looked very similar to mu and according to the OCD at least might have technically had a slightly different sound than sigma (I dunno, I ain't a linguist), and there was some weirdness in early Dorian scripts about san and iota, which initially looked very similar to Ionian sigma. By Herodotus' day san had dropped out of most scripts as an independent letter and had been replaced by sigma, but in the Dorian dialect the letter was still called san, even though it was written identically to Ionian sigma in most cities and had the same sound