This is a Water Tap Key and a nut (should you need it) to fix an old Tap or when the key has gone missing.
I think you find these kind of tap keys only in Europe but check the drawing with the "Thingiview" tool to check if it will fit yours.
Solidworks files attached for your tweaking
I printed the part from PET-G to be strong and gave it like 90% infill. Maybe an overkill but so far it can move the tap wrench just fine.
I broke mine at home so I had to find a replacement key or buy and replace with a new Tap.
Well, thank God for 3Dprinting !