From the scholia to Aristophanes, Knights 603, where the headword occurs (web address 1); cf. Clouds 122 (web address 2) and (echoing Knights 603) 1298; and Athenaeus, Deipnosophists 11.467B (11.30 Kaibel).
On horse-branding see generally kappa 2055.
[1] On san see e.g. A.G. Woodhead, The Study of Greek Epigraphy (edn.2, Cambridge 1981) 15-16. In appearance it was like a sigma rotated 90 degrees to the right (producing a result akin to a modern capital M).