Thanks for the replies
It does seem to be made to stretch something open (leather, cloth, animal mouth), and/or to hold something in an open position (sprinkler head as Andy suggests)
The circumference (around the jaws and the black knob) varies from 105mm to 116mm depending on the position of the lock bar. About 113mm in the position pictured
I am puzzled by the heavy cast brass construction (it weighs 930gm) which would make it expensive to make and too heavy for a tradesman to want to hold for long periods. Its design seems almost decorative. (I'm ignoring the suggestion that its a giant roach clip)