SWR nailed it! I have used these many times in the long ago years, when I used to do oxy/acetylene welding in auto repair. Waaaay back in the early 70's ..........NGE
I have a couple Acetylene tanks and this wrench is too large for any that I have seen. Looks like the stove wrench posted but it could be for anything.
Shut-off key. As Silver Searcher said: Could be gas, could be water. Any gate, barrel or ball-valve controlled flow. We had similar things in our church (c.1900 building) on the radiator valves and the sexton (me) held the key so the rabble couldn't turn the water heaters on and off. That way everyone got to complain equally.