First, the pulley and gear that DHector photographed is off of the Atlas 3950 lathe and has the small spindle gear on the opposite end of the pulley from where it is on all of the Atlas mills and other lathes. It has a Clausing style part number. The 3950 is the Atlas replacement for the 618, which went into production in about 1973. We don't have the drawing for the M6-79 spindle pulley & gear off of the 618 because it is still available from Clausing.
Second, background on Atlas part numbers prior to 1957. The first or first and second character ID'd the first machine that the part was used on.
9- 9" Lathe
10- through 10F- 10" Lathe (10 through 10F)
Ln- (where "n" is a single digit) Mostly Craftsman 6" (101.07301) and 3/8" bed 12" plus some wood lathes.
M6- Atlas 612 or 618 6" lathe (why M6 and not just 6, no one alive today seems to know)
M1- Atlas mills (most parts from M1 through MHC)
MF- Atlas MF mill (MF through MFC)
MH- Atlas MH mill (MH through MHC)
S7- Atlas shaper
Following the hyphen were one to three digits which if you know how to read it identifies the noun name of the part. For example, "31" is a spindle, or the most significant spindle in the machine. So 9-31 was the spindle for the 9" compound drive lathe. 10-31 was the spindle for all of the babbit bearing 10" and 12" lathes. M1-31 the spindle in the mill. M1-31T for all of the Timken equipped 10" and 12" lathes, etc.
With the exception of "T" for Timken and "G" for Gear, a suffix letter starting from "A" indicated a modification which may or may not have been backwards compatible. There are half a dozen or so exceptions to the forgoing on the mills. All of the parts having to do with the table power feed should (and originally did) begin with MF. But a few parts instead of for example becoming MF-14A became M1-64. Despite the fact that the Kickout is not use on any M1 model
But in short, except for a few screwups, once a part number was assigned to a part, aside from modification or the Clausing Conquest, the number never changed. So the parts lists for the mills include a few 10-. L2-. M6- and S7- parts.
The original three mill models were M1 (hand screw driven cross feed, in-out feed and table lift), MF (power or hand screw driven cross feed) and MH (lever and rack & pinion driven cross feed and table lift). All mills shared a common serial number pool which ran from 000001 through probably around 012500 (final number not known and this does not include the maybe 1000 or so Craftsman 101.15500 (MFC) sold by Sears). The M1, MF and MH ran from 000200 through 001344, M1A, MFA and MHA from 001345 through 005465, M1B, MFB and MHB from 005466 through 008123, and M1C, MFC and MHC from 008124 up. The highest number currently known is 012304.
The only two readily apparent differences between the base, A, B and C models is that the base (unlettered) models were 16-speed machines with 4-step countershaft and spindle pulleys, A were 12-speed with three step pulleys and B and C were 8-speed with 2-step pulleys. The apparent differences between B and C were that C had no upper cutter guard (also often missing from the earlier ones), the Arbor Support (and another part numbering mistake) was a little longer to attach an Arbor Support Bar (also often missing) and the Cross Slide Screw Bearing is about twice as long as on the B and earlier, most of that length at one diameter. The increased length was where the lower end of the Arbor Support Bar attaches. So if the Bar is missing, the bearing will still be there. Note that the drawing of the bearing in all of the mill manuals that cover the C models is actually of the earlier shorter one.
To further confuse the issue, it appears from the catalogs that Atlas didn't decide to call the A models "A models" until the B models replaced them. They apparently never made any nameplates with an "A" on them. Plus the older pulleys apparently went out of production shortly after each change. So it isn't unusual to see a base model with 3 or even 2-step pulleys. There were some other more subtle changes that may help if the nameplate (with serial number) is missing.
