I have had some of these in my Walden collection and often use them. I was missing this 1/2" size and picked it up on E-bay. It has a split head which is slightly larger than 1/2", you place the bolt/nut in it and slide the sleeve down to lock it in. The second picture shows them in a 1954 catalog. They also made them in a long shank model, although I have never seen one in the wild. Last picture shows it holding a 3 1/2" bolt.