I have previously proposed that Gruen Guild Factory T was Chs. Tissot Fils of Locle--successor of Charles-Emile Tissot. In Dec 1917, the firm was renamed Fabrique d'horlogerie Chs. Tissot & Fils SA.
Sharing evidence. Note the particular shape of the balance cock scallops. Compare to Gruen calibers 145-149 & 266-67, all of which Cleves tables say were introduced in 1920.
Tissot-signed example, circa 1921-22, of what would be the Gruen cal 267 (17J variant of the 266, image just above). This initials/triangle/ADOC mark registered 12 Oct 1921.
Tissot design from 1916 --> lapsed 1922
Tissot design from 1919 --> perhaps a relative, but not clearly legible; renewed until lapsing in 1934