I don't know. I think that may just be a opposite face to the main face that suggests this device is designed to slide along something like a 2 x 4 or a steel rail. The spring keeps the adjustment screw tight. The gap angle that can be set by the adjustment screw is very small. It is on a fairly clean workbench and there is sawdust on the tool, so we can assume that it is a carpenters tool and not a tool that is used on a railroad or in a machine shop. Beats me.
Looks like some sort of tubing clamp. Like you would hook the curved part on the left around the tubing and feed the end you want to clamp through the space in the jaws on the right side and twist the thumb screw to tighten it down.
Indeed, but the hook certainly looks to be for hooking on something, w/ the spring end acting/working on the "cylinder" being held by the hook.
