Looks like they might be a variation of this style clamp? Are you sure everything is there or is it possible there are parts (wood jaws or a secondary stretcher) missing?
I added a smooth secondary stretcher and drew in some boards being clamped. The metal arms would swing outward as clamp pressure is applied. This in turn would pull down on the secondary stretcher, effectively putting pressure from all four sides.
Just my best guess as to what you may have there.
Last edited by Dave McCann; 07-11-2020 at 12:05 PM.