I don't know what the correct name is for the mechanism on the back. We could call it the feed select mechanism (FSM). I think the chances of you finding one for sale are about zero. If you could find a No. 1/2 that someone is parting out, maybe, but 1/2s are rare as hen's teeth. The feed system from a No. 12 could probably be adapted to work, but it uses a separate drive motor.
I'm sure you could make something similar to the original system, but you'd be mostly working from photos. I would use V-belts or automotive serpentine belts if I were going to do that.
Here’s how the rest of the feed system works: The two belts and 3 stacked pulleys drive the FSM. It has a gear train that I think is just used to reverse the feed direction (there doesn't appear to be any other way to reverse the feed). The cone pulleys on the back of the FSM turn whenever the spindle is turning. I don't know if the FSM has a neutral position that can disengage the feed there or not. If not, its output, which drives the rear end of the telescoping shaft (via a universal joint), is always turning as well.
The other end of the telescoping drive shaft has another universal joint which drives a worm gear that rides in casting which is hinged at the rear. When the feed is engaged, the worm is brought into contact with a worm-wheel that directly drives a sleeve in the top of the saddle, with the nut for the table feed screw. When the feed is engaged, the nut rotates and pulls the feed screw through the nut, moving the table.
Here's a photo of the bottom of a No. 1/2 saddle:
The saddle is siting on it's rear surface. You can see the worm gear casting tilted out from the saddle, pointing towards the camera. Immediately above it is the bronze worm-wheel. The worm casting is normally rotated up so that the worm gear engages the worm wheel. On the right end of the shaft is the gear that's driven by the right hand-wheel.
Here's a photo of the top of the saddle:
You can see the tube (hollow shaft) that moves the table. About in the middle of the saddle, on the left end of the shaft, is the nut and backlash adjuster. The bronze worm-wheel and drive gear can be seen to the right. The leadscrew runs through the nut and tube. The feed mechanism moves the table by rotating and pushing/pulling the leadscrew and thus the table.
Here's another photo of the underside of the saddle, with the saddle resting on the table:
You're looking at the back of the saddle and table. On the right, the right angle drive for the right side handwheel is visible (it was removed in the first photo), with the shaft for the handwheel pointing away from the camera. The large round black thing on the right is the cover for the gear which drives the smaller gear shown in the previous two photos. You can see the stub end of the shaft that drives the worm gear peaking out from under the right angle drive. There's a universal joint that attaches to the shaft and in turn connects to the front end of the telescoping drive shaft.
I'm not sure how the lever that engages the feed by tilting the worm gear casting works. I don't have any good photos of it. Maybe you can figure out how that works and post some photos of it? As always, lots of good photos are greatly appreciated.
