Atlas made the lathes for Craftsman and at the very end, I believe Clausing was involved. They are very capable machines and very easy to fix.
Some of the South Bend, Clausing, and other brands used a 2 1/4-8 spindle nose thread. These machines typically could take a 5C collet, while the 1 1/2-8 were relegated to a smaller MT2 collet.
A lot of chucks were flat backed with no mounting system and would bolt onto a plate designed for a specific spindle nose such as D1-3, 1 1/2-8 threaded, or L00 taper. They could make the complicated part (lathe chuck) in one SKU and adapt to many lathes with the cheap, changable backing plates. Smart design.
If you want any help with that lathe or chuck, ping me. I've rebuilt several of the 6" and 12" lathes. Might have some usable parts or could answer questions. I have chambered and threaded rifle barrel blanks on the Craftsman 12x36 I had. They can be very effective, when you understand where their limitations are.