Shown below is a rare Sheldon lathe from circa 1958, a 13-inch M Series model with the Serial number 22925WM - the "W" indicating that this was example was supplied mounted on an underdrive cabinet stand. This version is normally found fitted with a Merkle-Korff Gearmotor unit that consists of a push-button-controlled electric motor that opena and closes - using a chain drive - the expanding and contacting pulleys of the variable-speed drive.  However, although the model shown has the same type of mechanical variable-speed drive with a wide V-belt, the control is manual, with a red-painted handle emerging from the front face of the cabinet above and to the right of the door opening. Connected to the handle is a long rod that engages with a flat-faced jockey wheel; pushing the handle down or lifting it up causes the jockey pulley to engage against two V-belts that, in turn, drive a shaft to move yet another shaft that opens and closed the variable-speed pulleys. Although having the appearance of a nightmare contraption, the system is reported to work well.
Sheldon obviously had second thoughts about the installation, and replaced the manual system with the electrical one mentioned - though unfortunately the motor chosen by Sheldon was to be phased out and today there is, apparently, nothing to replace it (at least the 'walking stick' mechanism is repairable and uses just simple parts).
The owner, Bill Price writes:
This Sheldon "walking stick" together with a 12" Clausing 100 were located through a Craig's listing in East Texas. Both, tucked away in a corner of a maintenance building, had been purchased by the owner's father at an estate sale 'several' years before - thinking one day he would 'fix them up' and, of course, there they languished gathering dust and becoming ever rustier.
They had, apparently, been a shop at some point and, as is so typical, the operators did not know what an oil can was and both needed considerable work. The unusual Sheldon intrigued me, so I restored the Clausing first and sold it in order to save the Sheldon for what seemed to be the promise of a long and probably challenging rebuild. The work started on the first of May, 2012 and was pretty well finished by the end of Aug 2012. I'm retired, so I was doing something to it on most days, a build like this can eat up some hours!
Although on teardown many problems came to light, two main areas were of special concern: the power-feed worm and wheel assembly in the apron was badly worn (and there was no replacement to be had) and the total mystery of the variable-speed drive system.
My friend and I ended up in his homes workshop making a one off worm/gear, making our own one-time use gear-cutting hob - which performed nicely.
The underdrive mechanism turned out to be just a matter of just taking it apart, noting disassembly and figuring its function and worn areas and parts. If memory serves, it has 12 bearings (of which 9 had to be replaced), 4 bronze bushing (2 needed to be remade) along with 7 of the 9 belts - it is quite a mechanism. On re-assembly getting the 100 lb Worthington drive assembly back into that small cabinet was a struggle and it took a friend any myself about and hour-and-a-half of kneeling and almost standing on our heads to wriggle it in and get all those belts, shafts and pulleys aligned.
I installed a VFD to run the 3-phase motor but in the event the variable speed output from this was disappointing and I was pleased to find that the original mechanical 'walking stick' system worked quite well, thank you! I have the impression the underdrive system - the motor, Worthington drive, 'walking stick' mechanism and jack shaft between the WD and spindle, etc., are going to need to be pretty closely watched, to maintain adjustment and keep the several lubrication points serviced. 
I have been using the lathe for three weeks now and am favourably impressed. The controls on the lathe and its appearance are very similar to those of many contemporary makes - South Bend, Atlas, Logan, etc, but my impression is that this Sheldon is probably a couple steps above them - a pretty beefy job..