The Sears 101.28990 is the same except for badges and plates or decals as the Atlas 3996. All of the taper attachments that Atlas made would fit all of the 10" Atlas and 12" Craftsman that they also made. The only significant difference between the 10" and the 12" in any given year were the swing, headstock casting, tailstock casting, and compound swivel. Some years the bed legs were different because of some strange ideas of someone in Sears but from 1957 on, either the guy had retired or he had finally given in and they were all the same except for Model Numbers and badges..
The Atlas Taper Attachment model numbers are:
700 (sometimes referred to as the Toolroom ...)
762
6822
For some strange reason, I have never come across the Sears model numbers.
The mid and the late Taper Attachments are interchangeable except for a couple of brass parts added to the final version, and will all fit either badge machine. You will find drawings and instructions including parts lists of the mid and late Atlas numbered ones in Downloads. The very first one (which almost never turns up anyway) we only have the instructions for. Last time that I checked, Clausing still had the added parts on hand if you manage to acquire the earlier one and want to upgrade it. The earliest version that Atlas made was called the Toolroom Taper Attachment. The others dropped "Toolroom" from the name.
FWIW, access to Downloads requires Donor status, any level.