First, it's bronze not brass.
Second 2 parts are missing, the tap bolt & the shoe that rides the end of the screw.
The tap wire of whatever length necessary is secured to the clamp and transformer or another clamp in the case of jumpers.
After that you keep yer damn hands off the device unless you have 2 feet on the ground.
Hotstick goes in the hole of the clamp body to swing the clamp into place hanging on a primary. Stick is pulled from that hole and indexed into the loop of the screw and the screw is tightened. Connection is made and stays made for years.
These were very common in the time of 4160 volt primary when a 3 foot wood stick was considered safe. They are still used but stick technology has changed considerably.
Billman the clamp you pictured is a bus bar tap. With the screwdriver handle it is most likely to be used on one end of a set of personal safety grounds to provide protection in a switching cabinet.