The fire hydrants I’ve seen use 5-sided ‘shafts’ for the valve control. A normal open end wrench, or even a 6 or 12 sided socket won’t fit it. A crescent wrench with parallel ‘faces’ won’t fit. (A pipe wrench will work, biting into the shaft).
This keeps the ordinary citizen from fooling with the hydrant.
A 5-sided ‘socket’ on the spanner will fit perfectly.
I agree there are different hydrant tops but most my city are square., but we kept different size scanners it wasn’t like there were infinite sizes.
You get caught using a pipe wrench on a hydrant it’s a $500 fine.