An indexing stop like this is for producing repetition work, and from time to time designs directly or adaptable for Myford, Warco, Boxford etc lathes appear in the model-engineering literature.
Industrially they were typically fitted to capstan lathes but can be used on conventional ones as well.
The mounting would vary from machine to machine, but typically the rotating part is on the headstock side of the saddle, and its adjustable screw-heads stop at a block whose own position is itself adjustable, along a bar mounted on the bed somewhere between the front shears and the lead-screw or feed shaft. On a full repetition-lathe I would expect the stop to be indexed by the capstan or turret.
A similar stop is used on turret-type bench drills, though built into the head and indexed by the turret's own stepping. This one might be from such a machine.
It looks as if the previous owner had obtained this specimen incomplete and second-hand somewhere, and has had to improvise a bit, judging by that long screw with a rivet-bush on it!
Edited By Nigel Graham 2 on 05/07/2022 13:26:32