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.