Key sticking or stuck? New are NLA. Most used are worn out internally so new tumbler won't fix them. Be careful buying on eBay as many are worn out internally.
Push them over a hose and they act like a sealing end cap. Turn them around and push into a hole to act as a plug. Cut in half - use one piece for a splice between hoses.
For most 4 cylinder, 6 cylinder and V8 engines from 1963 to 1985. Made to remove the race on the end of the crankshaft when replacing the front crank seal.