Can't help, but...it doesn't just compress it, it then holds it in place against serious pressure to turn it loose (thus the Vise-Grip type arrangement). That inclines me to think that you're right about the thing being compressed being a spring.
I'll follow along on this one, waiting to find out what it is. For bellows springs on a pipe organ...or the front axle hairpin spring on a 1938 Henway Light Four...or something like that.