The primary headword adjective kausteiro/s (LSJ at web address 1) appears in twice in Homer (Iliad 4.342, 12.316) in the feminine genitive singular, used figuratively of battle. The masculine appears by analogy in late Epic: kausthroi=o (sic) in Oppian, Halieutica 2.509 (deriving kausthro/s from kausth/r). The grammarians spell the masculine adjective with ei, to match kau/steira. Grammarians from Herodian on (De prosodia catholica vol. 3,1 p. 199) defend the ei spelling as a Boeoticism (ou) ma/xetai: "it cannot be disputed"). [Adler reports that mss GVM of the Suda read kau+teiro/s here, and that ms F has kau+thro/s.]
The second form, kauthro/s, would be an alternative for kausthro/s, like the alternative forms kauto/s and kausto/s; but the form only appears in lexica (kappa 1143, and ps.-Zonaras s.v. zei/dwros, in the derivation of kausteiro/s).