Definitely not fire hydrant, as it isn't long enough. I've turned one or two of those in my time as a vol. firefighter. Those stems can be stubborn to get broke loose. I've also broke a couple stems, due to the water dept. not servicing the valve as they should have allowing the stem to seize up. They are also, as mentioned, pentagonal in shape to deter water thieves and vandals.
I'm in the gas line work group on this one, as the slot could very well be for turning off small meter valves. Still not sure what the small range jaws would be for, though.