I picked up this very narrow-throated C-clamp recently. One, because I can't resist clamps and two, because I was unfamiliar with the name.
Not surprising. "Simanco" is short for the Singer Manufacturing Company, the sewing machine folks. I've discovered it was intended for use with the Singer Hand Crank Pinker and the Singer Sewhandy Model 20 Child's Sewing Machine, both of which needed to be securely fastened to a table top before they could be hand cranked effectively. Now it has joined the old toy Singer sewing machine in my study.