To flesh out what I wrote about the Sears versions of the Atlas 6" Mk1, the actual 618 was made from 1937 to 1972 with Timken bearings and 1"-10 spindle nose. The reference several places on the Internet about the early 618's having 1"-8 spindle nose threads is simply wrong. The 618 never and anything but 1"-10. And there was no change to the 618 spindle in 1958. The confusion is between the early Atlas 618's and the Craftsman 101.07301, caused in part by people erroneously calling it a 618. For whatever reason but no doubt because someone at Sears insisted, the 101.07301 alone had the 1"-8 spindle threads. It was made from 1939 through mid-1957 (not 1958). The differences between it and the Atlas 618 are (or were originally) only in the headstock, bed and legs. It had Oilite sleeve spindle bearings. So the rest of the spindle was also different. And it had a ball thrust bearing similar to that used in the Atlas 9" and in the 10" models supplied with babbit bearings. The headstock casting, cover, spindle gear spacer and spindle pulley bushings are also different. The bed was different (because it had to mount a different headstock). And the legs were different (because Sears wanted them different). The remainder of the headstock and all other parts on the machine were when the lathe came out the same as on the 618. I haven't yet pinned down the year, but possibly as early as 1940 Atlas made the 3rd and final change to the countershaft assembly on the 618 (the first version attached to the headstock instead of to the bench or stand and was only made for a couple of years), It changed from having a short bracket with the countershaft hanger sticking up to a taller curved bracket with the hanger hanging down from its pivot point on the bracket (quite similar to the one used on all of the 10F's). For whatever reason, the same change was never made to the 101.07301 (although they are often found today with the later countershaft, at least in part because an owner somewhere over the years acquired the lathe without a countershaft and bought the later version either because he wanted to or because it was more plentiful. Atlas ceased production of the 101.07301 in mid 1957 and replaced it with the 101.21400. The 101.21400 is identical to the Atlas 618 except for having Craftsman badges. So it is at least approximately correct to say that there are two versions of the Craftsman Mk1 - the 101.07301 and the 101.21400. And the earlier model was the one that had 1"-8 spindle nose threads. But it is totally incorrect to call it a 618.