1/4-inch Compression Bolt (a.k.a. "Button Head") removed from the wall at Sespe Gorge. Amazingly, this bolt was pulled out of the wall by simply sliding a nut tool under the hanger and using it as a lever.
Surprisingly (to me), this is smaller than the dozen or so other compression bolts that I have removed. It turns out that the previously removed compression bolts were actually 5/6-inch (or 8mm). I had mistaken those previous removals for 1/4-inchers. I had not even been aware that a 5/16-inch variety existed.