Actually, the Timkin bearings came later. I don;t believe that they were ever fitted to the original 9" models. They became a slightly higher priced option shortly after the first 10" came out. And on the 12" that Sears sold, about a year after the first two 12" models.
The numeric part of the model numbers of the Atlas 10" and most of the later later 9" models was the nominal bed length. Originally, the 10" and 12" were available in 4 different bed lengths, 36, 42, 48 and 54 inches. The headstocks were about 18" LOA. So a 54" bed length yielded a 36" between centers. The 36" and 48" beds were discontinued circa q945 or early 1946. The QC models didn't appear until 1947 so officially there are only QC54 and QC42 models from the factory. Also discontinued at about the same time were babbit spindle bearings and vertical countershafts.