Today, while removing the copper conductors from some high voltage ceramic insulators, I unscrewed this black isolator from the top of one, and when I looked inside, I was greeted with the sight of a small round copper item with a U stamped on it. The blank and red plastic sleeve fit snugly in a hole within the threaded shank of the isolator. The hole that accomodates the large copper washer is about 0.87 inch deep.
I thought it interesting enough to post here. There appears to be no triggering mechanism so perhaps the blank is fired when heat becomes too great. Perhaps there are drawings somewhere but I did not spend alot of time looking.