I have seen those type of wrenches with 8 sided large openings before, but, still have not found a hint what it fits either. There were hose fittings way back that were octagonal, but a ring spanner would get stuck if you used it on a hose, so it has to be for something you can get it off of afterwards, like a valve, or a nozzle...kinda large for a valve, seems small for a nozzle, dunno..
(I have looked at old fire hydrant pictures, some had weird and bizzare fittings, but nothing like a 2 1/2 inch octagon..)