Front wiper arm replacement

Evening all,
Had to order a replacement near side front wiper arm for the P38 after bit of an accident with the last one.
Anyway whilst waiting for the new one to turn up I cleaned the splines up on the motor in preparation for the new arm.
Well the new arm turned up this afternoon but to my surprise it doesn't have the groves machined into it, its just a tapered hole.
Should it have the groves in the hole or will it cut its own as I tighten it down?

Granted it wasn't a genuine part but there was no mention when I ordered it that it would be of inferior quality (I did try to order a genuine part from my local dealer but apparently there not available anymore.

regards

Dave

hammer?

as you tighten the nut, its bites onto the groves in the shaft. dont matter where you order them from or for that fact, what marque of car you have, they are all the same