Jack,
Some eBay sellers are clueless and don't really know anything about the items that they sell and don't bother to find out. AFAIK. Atlas never put a Model Number on any accessory. Numbers in castings are the part numbers of the part made from that casting. But 90% of Atlas accessories advertised on eBay are listed as though the Atlas part number that they use in the ad is actually a model number. Atlas's part number assignment SOP in the 1930's and 1940's was that the first one, two or three characters (always followed by a hyphen) identified the first piece of equipment the part was used on. The identification wasn't always obvious or logical (L3 was used on Craftsman 12" lathes and M6 on Atlas 6" lathes for example - "L" could be for Lathe and "6" for 6" but what the "3" and "M" stood for who knows). But part numbers beginning with "9" first appeared on the 9" lathe. "10" on the first 10" lathes. Etc. If the part continued to be used as is on subsequent machines, they unlike GM didn't assign it a new part number. The Atlas mill has a number of M6- and S7- parts, for example. And my Atlas 3996 12" made the Summer of 1980 still has a few 9- and 10- parts in it. Atlas model numbers don't follow any of the part number conventions.
So all that you can say about a part number that begins with 10- is that it started life on a 10". If the steady rest you are looking at is the two-piece style that I think I found , you could ask the seller what the part number of the lower part (that sits on the bed) is. I can't make it out in the photo but if it is 050-119, it is for a 12".
There is a guy making steady rests for the Atlas 10" and 12" and offering them on eBay @ $150. They look pretty good so far as what you can see in the photos. I think the basic design approach is from one designed by one of the Atlas competitors.
One general eBay comment I'll add (doesn't only apply to steady rests). Anytime you see an accessory advertised as being for Atlas, Logan, South Bend, etc., be leery. Odds are that it only works on one of those makes. Or may not even be for any lathe.
Robert D.
