I'm sure this has something to do with grounding...

[Frank]

What is it and how is it used? It appears to be made of brass/bronze.

[john k]

I've seen power company linemen use these to hook one wire to another from the ground, turn the thumbscrew to make the connection.  Someone should have the correct name.

[rusty]

"Hot line clamp"

[Aunt Phil]

good old fashioned stick bug

[amertrac]

thats what is is.  all done on hot wires using a hot stick I have several  hot stick tools from my days with the power company.
bob w.

[amertrac]

pics are of a hot stick limb trimmer after some modifaction
 64longstep is responsible for the forging work   

[amertrac]

this is work in progress a hot stick wire raiser

[Fins/413]

That would make a heck of a pike.

[johnsironsanctuary]

Found this in a Rose Tool post of aCrescent Catalog from the thirties or forties.

[Branson]

Picture #1 shows
KEARNEY TOOLS #7296 VINTAGE TUBE, PIPE, CABLE CLAMP

Picture #2  shows a current Kearney grounding clamp

[Aunt Phil]

So, in vertical order you got a porkchop
a stickbug
a bar or cabinet stickclamp

BTW, the only time a porkchop instantly releases is when you don't want it to.