Yes a jig for cutting spiral flutes. Notice the cup engaged with the work piece. As the crank is turned two things occur.Found in the father in law's workshop. No clue what it is for but I swear it looks like the sliding part is to mount a router but it was lying next to the lathe.
I have not used one of these but it is my understanding that this fixture/jig may have been used with the piece still in the lathe. The other end of the work piece would be centered and supported by the headstock, while the jig frame, served as the tailstock. The lathe was NOT turned on during this operation.I fear it may be missing something as I see no way to mount the workpiece on the end opposite the crank.