Gene,
To add to or comment on what Pierre wrote, I have about a fourth of the Atlas catalogs between #1 and #40, beginning with #5, with the mix somewhat weighted towards the later end. I also have later ones but these cover the original 9" through the advent of the 10F. In these catalogs, I've not seen any indication that power cross feed was an option. There is some confusion caused by the way in which they did the most recent dated 10" parts list that I know of and reading it, one might assume that you could buy a 10F without it. I think, however, that it was an attempt to include some coverage of the 10D which was still available for an unknown time after the 10F appeared. As far as I can tell from the catalogs, if the lathe has a 3/4" dia. lead screw, it has power cross feed and it is a 10F. And if Popular Mechanics ads seem to belie this, I think it was probably because the 10D and 10F availability overlapped for a year or two.
The first 10" to appear in the earliest catalog I have that shows them were 10A through 10D. Although strangely. in the catalog it never actually describes one of them as a 10D. The A through C were stripped down versions called Unit Plan lathes if Atlas 10" or Universal Lathes if Craftsman 12". The A consisted of the bed (and feet), the headstock, the tailstock, and a compound that clamped directly to the bed. No carriage, rack, countershaft, leadscrew, change gears or back gears. The B added a few things, the C a few more, and the D had everything that was available. The model numbers of the D were 1036, 1042, 1048 and 1054. The A, B and C were the same plus a suffix A, B or C. About the time that the 10F came out, they dropped the A, B and C and added the E which was as nearly as I can tell a C without the countershaft. Its model numbers were 1036E through 1054E.
When the 10F came out, the model number style changed to H or V36 through TH or TV54. So if you have a TH42, it is or was a Timken bearing 10F with horizontal countershaft and could have been made as early as about 1939 or as late as 1957.
On the part number question, once a part number was assigned, it never changed until the part changed. 10D-6 was used on 10" and 12" lathes up until 1957. My 3996, built Summer of 1980, still has a few 9- parts in it.
Robert D.