I suspect this to be the tailstock for an Atlas milling machine indexer.

It  is missing the handle on the handwheel, and the clamping screw and nut for the sindle lock,
 and it looks a bit rough, but it's in pretty nice shape under the grease and crusties.

There is a number on the handwheel, M6-23, in raised letters on one spoke.

It has a MT1 cutaway dead center in it.

 Center height is three inches.

 The alignment lugs are offset from the center by 1 inch.

Cheers
 Trevor Jones