Lee Kim:
Take a look at the weld (or brazed) bead on the side of the vise. The weld (or brazed) bead runs between the two drilled holes on the slide or ram of the vise. Someone used that vise hard, maybe beating on a job held in the vise using a sledge hammer. The vise broke across the slide or ram. Whoever did the repair may have jigged the two broken pieces together by drilling the small holes and temporarily bolting a piece of steel spanning the break. This held the broken parts in position until the weld could be made.
The vise is beat up and well worn. The moveable jaw and slide (or ram) are likely made of a cast iron alloy, possibly containing a percentage of steel. Once broken, a repair by welding or brazing is not the most reliable thing. Cast iron alloys, once broken, can be welded or brazed, but depending on that part to have the strength of the original unbroken casting is not something I'd do.
The vise has the single long screw sticking out the bottom. This type of vise was meant to be mounted on anything handy, often on a temporary basis. Usually a nut with a pair of "horns" or handles was furnished, as was a large bearing plate or washer. Drill a hole in a bench top, saw horse, or other wooden surface such as part of a building under construction and clamp the vise to it with that large nut. If you needed the benchtop cleared, or were done at that jobsite, you took off the vise by means of undoing that one nut, no tools needed. Plumbers, pipefitters, and other mechanics who came to jobsites brought that kind of vise with them as a portable tool.