Quote Originally Posted by Anscojohn View Post
The M-42 lens mount was originally from Praktica. Asahi had been copying Praktica lens mounts from the get-go--first the smaller diameter, then the M-42.
As far as I am aware, the 42x1mm mount first appeared on the Contax S and D in 1948, followed by Edixa in the early 50s. When Pentax later adopted the Contax/Edixa thread it became known as Pentax (inaccurately).

Praktiflex was I think a 40x1mm thread (1938) but I do not know if earlyAsahiflexes were the same. I know they were close, at 40-41mm, but I have not tried interchanging lenses.