But if you have also a Follower Rest, you will see three different part numbers on the three cast parts. They didn’t put part numbers on machined parts.

The numbers visible on cast parts are the casting number and finished part number of the casting. The only things that Atlas build that ever had the full assembly model number on them were the nameplates on the lathes, mills, shapers, etc. The only difference over the years was the paint color. There was only one 6” steady rest and one 6” follower rest ever made. I already answered part of your question, but not all of it. And it took a while to get close to caught up. Just last night was I able to download mail from groups.io. On the 26 th, my main machines mother-board failed and although I had a backup machine (a laptop), didn’t have everything on it. If the bright blue color still looked relatively new, my guess is that the Follower Rest was probably made after 1977, which was when Atlas changed the color of the 10100 (Timken bearing MK 2) from machinery gray to bright blue. M6-396 is the casting number and the part number of the main casting. So assuming that the number made in each of the 18.5 years was the same, and 31000 were made (based on the highest reported serial number of 030965) your machine would calculates out to 22 November, 1954. Clausing has no production records on the Atlas built lathes.

Unfortunately (for us), Timken did not engrave a date onto the 6” spindle bearings as they did for many years on those used on the 10” and 12”. And judging just from the one catalog photo, probably it, too. So any Atlas built Steady or Follower Rest will fit any Atlas built 6”, except maybe the 101.07300. And its only difference is that it is shorter. All of the Atlas built 6” lathes (except that we aren’t absolutely certain about the 101.07300 as we have no parts list for it) use either the same bed or at least the same casting with varying hole patterns for legs and rack, except for the 612.

Atlas lathe information