The 6¡± Craftsman metal turning lathes (made by Atlas) had 1¡±-8 spindle nose thread until 1959 when the Craftsman changed to the Timken tapered roller bearing headstock. Then the spindle nose changed to 1¡±-10. From 1940 through 1958 the Craftsman 6¡± lathes had the bronze bushing headstock. The first Craftsman 6¡± metal turning lathe which came out in 1938 had a 3/4-16 spindle nose. You are correct stating that the Atlas 618 lathes always had Timken bearing headstock with the 1¡±-10 spindle nose. This is based on a review of Craftsman catalogs that I have on pdf.
Steve Haskell
Troy, MI
Steven H