I am no expert but to me that looks like a purpose made (or purpose adapted) vise maybe starting with a (broken?) blacksmiths vise. I would propose that it was made to slide into a groove in some fixed surface then locked in place with the lower screw, binding the fixed surface between the lower screw plate and the shoulder in the fixed jaw. The only problem with that is it looks like it would have potentially limited the travel of the active jaw. The curved arm of the anchor screw could be a bent and shortened remnant of an original leg. Otherwise, I don't see how it would have been mounted. There seems to be a lot of wear in the area between the hinge and the collar on the jaw which would be consistent with such a use. I know that doesn't help much. Hopefully someone has a better answer.