Vince,
OK. When I wrote earlier that 101 told us it was made by Atlas and sold by Sears, I forgot that Atlas did make one lathe whose model number began with "101". You have, as you just wrote, a Model 10100, which is a 6" Mk2. Sears sold the same machine under the Craftsman badge as their Model 101.21200. There were two variants, the Model 3950, made for about a year, and yours. As I wrote earlier, both use tailstock assembly 3950-24.
The earlier Atlas 6" (which some people began calling the Mk1 after the Mk2 came out) was the Model 618 (there was also a 12" between centers Model 612 but they almost never turn up). They all used the same tailstock assembly, M6-5X.
Although the main tailstock castings are obviously different between the two models, some parts were the same. The base is the same. The rams and the ram screw bearings are the same. The ram screws and the handwheels have different part numbers. If you only need a ram, any for Atlas 6" will work (except the Metric one). If you need the screw, I would stick with looking for one described as being for the Atlas Mk2, Atlas 10100, or Craftsman 101.21200. And same with the handwheel.
Robert D.