There are sometimes weird proprietary items you find in machines for sure. I've never seen or read about a 9 TPI leadscrew, what lathe is that?
On my Elgo KL26 Capstan lathe, that I'm converting to CNC, manufactured by the firm Elgo S.A., from the town of Elgoibar, in the Basque region of northern Spain, the parts manual lists a 5-inch chuck for it as an accessory. take a look at the photo and see if you have any ideas on mounting a back plate on this spindle. The spindle threads measure 2.8215 and is 12 TPI with a native 5C collet taper in it. There is not a registration area in the rear with a 90 degree stop, but there is in the front, it measures 2.1675. The rear area measures just over the threads (2.834) enough to allow the nose thread protector to ride up the spindle about a quarter inch.
On the gear train on the 1912 12-inch lathe, from a photo, I believe I counted 30 teeth on the reverser. The old 11-inch or the 13-inch, can't remember right now, had a 30 tooth on the spindle and the reverser, I asked him to count them and confirm.
