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.

I have a 1/4" one in my toolbox.
-Don

That's a cool device.

Hello, Coolford. That is a great tool, first one I have seen!!! Thanks for sharing, Lou

Great marketing name a cool tool.  Really neat!

Hmm. I can think of a couple times that would have came in handy.