That is a stump vice used to hold the bar while sharpening a chain in the field. There should be a pin or a spike coming out of the bottom of the "U" but they always seem to get broken off.
I have one in the kit for the Comet that looks almost identical...look at yours again and you will see where the spike is broken off.
Well, you nailed it. I've had 3 Comets. Looking at my old pics, the ones that came with tools had this. If I recall correctly, one of the kits had a spare clamp, so I kept it in case I bought a Comet that was missing it.