That is a hybrid of different items mounted together for a mill vise.
First, that is a Palmgren vise bolted to the slide. The slide looks like a milling attachment from another lathe-not South Bend. Then the base with the dovetail looking round mount was added so it could be used on a South Bend lathe.
The slide could be from a Logan, Sheldon possibly, who knows. It's definitely not a OEM milling attachment. Definitely home made.