Picked this up today at the antique store, trying to find out some information about it.  The knob at the front adjusts the main screw, and when you're close you turn the handle about 90 degrees to clamp it down.  I'm not sure what the contraption on the back does. The screw is stuck, so I'll have to try to get everything freed and mess with it more tomorrow.  I've never seen such a thing so it came home with me.  I can't seem to find anything on the internet, hoping to find out if this is a factory made vise or someone modified it along the way.  Either way, I think it's neat and could be useful after some TLC.  

Got it all freed up and clean(ish) today. I'm going to guess that most of this was a shop built modification. Or it has numerous repairs. It works well, but only has an effective clamping range of about 1.25". The back contraption keeps the main screw from turning once you dial it in. She's definitely seem a lot of use. The sides of the sliding jaw are welded and ground to take up slop and the handle is almost worn through from something handing on it maybe? I'd love if I knew it's story, but probably never will.

Going to mess with it this week, to see if I can't improve upon some of the repairs and I'll get some paint on it eventually too. But in the meanwhile, here are a few more pictures.

over at garagejournel there's  vise forum and some real vise nuts that post there. You might find out something there about your unique vise