Hmm! If only I had a pound for every time someone has asked me that down the years. Being a wally, I wrote it in a yellow sticky which was in a box of bits but in the great move the box got binned along with the sticky...
They are only self tappers and go through the rear mudguard (the bit you've ordered) and into what I would call the rear mudguard (the black plastic tail end unit) which has holes/bosses with holes moulded in.
If you get down and dirty with a decent torch you'll see what I mean. A poky stick will tell you the length and a bit of Plasticine or BluTack will tell you the plain diameter of the three holes.
The stainless screws are really only an issue if you need to remove the thing in the future e.g. to remove your number plate if it is fastened with nuts and bolts as the mudguard thingy covers the top two nuts. When I tried removing the mudguard on my K12S (the only bike in known history which had the guard fitted as standard as it was a press bike) the screws just chewed up and trying to drill the heads out just melted the plastic...
