Toby you got it about right . Our American cousins may know it as a blacksmiths vice , The "leg" that also forms the base of the rear jaw fastens to the bench and goes down to the floor . Thus taking the stain of any hammering away from the bench . These were in common use prior to the paralel jawed engineers vice that is most common to day . They have ceryain advantages over the modern vice for gun work , but more to the point I was "brought up" on one so When I started working in Price Street thats what I wanted I'll try to put a picture up if I can work out how.