Aaaarrrrggghhhhhhh.
I finally opened up one of my model M's by brute force because I could not find a 5,5 bolt driver..... I bought 2 bolt screwdrivers via the internet : 5 and 6 mm just to be sure I could open it, but I finally found out it's a 5,5 : these are very uncommon in the EU.
Should have checked the forum or asked in a post. I ultimately opened it up by drilling the holes since I am going to use it as a key donator board anyway.
Still got a question : how can I properly remove the metal plate from the assembly? Drill the holes?Any ideas? My hope is that if I remove the metal plate underneath the PCB I only have to cut some plastic in order to get what I want, but if the switch is attached to the metal plate in some way I might be in for a nasty surprise ( Waisted a perfectly functioning model M )
EDIT : I found some useful material here :
http://geekhack.org/showwiki.php?title=Island:15277 . The guy drilled all the little holes apparently, I can see the springs have some kind of T-shaped plasticy thingy attached to it which is dangling loose. So I guess I have to somehow add something as a base to the switch in order for it to properly function. I guess nobody ever tried this before

How to best cut the metal?
[excuse me for hijacking this thread, but the question in the OP was already answered.....]
Regards.