Wiper wheel box - washers?

[itiejim]

I'm just getting around to refitting the wipers and have to put the wheel box nuts on - the one which sits under the wipers arm nut and secures the wheel box to the body. Can anyone confirm whether the nut should just have a large washer underneath it or is there something more bespoke!

[JensenA]

Pretty certain it's just a washer.