A hanging device sold by Rosco.
http://www.rosco.com/us/hardware/sureclamps.cfm
See also Clancy Clamp.
http://www.rosco.com/us/hardware/sureclamps.cfm
The Sure Clamp has a grip range of 1-1/4" to 2" and is used to hang lighting fixtures or scenery directly to pipes. An extra-secure substitute for the standard C-Clamp, it closes automatically when weight is hung on it and is further secured by a hand knob which requires no tools for adjustment.
See also Clancy Clamp.