Special tape. Put one on my s95 a few years back - its still there after lots of use. Will have to look into this for my M.
If they use VHB tape from 3M, it's pretty incredible stuff. I used some to attach an aluminum sidesaddle to the receiver of my shotgun (the bolts that came with it left it either too loose or pinching the receiver enough to reduce reliability of the semi-automatic cycling). After more than two years, exposed to extreme heat, extreme cold, lubricants and heavy use, it ain't comin' off. Doesn't even budge or flex. Love that stuff! Of course, I had to buy a roll (75', if I remember correctly), but I only needed about 6", so I'm always looking for a reason to use it.
Can you really take it off with dental floss?