It is a spanner to adjust a "bearing nut", which is a basically a thin nut with a round Outside Diameter that has a series of slots cut across for the hook of the spanner to engage. The curve of the spanner matches the OD of the nut.
The spanner doesn't have to apply a lot of torque, as the nut normally is used to hold the shaft from moving endwise in the bearing bore. The nut is locked by a washer that has a tab that engages a keyway in the shaft and has tabs around the outside. One of the outside tabs is bent over to engage in one of the slots that the spanner hooks into to lock the nut.