Mounting grooves and the slide "recess"

[Bateau]

Do mounting grooves work by just sliding accessories onto them?
So grooves positioned where the grooves under the barrel in this pic http://www.hkpro.com/uspmatchtarget.jpg wouldn't be able to mount anything?
What alternate methods of mounting accessories underneath pistol barrels are there?

http://www.hkpro.com/socomdiagram.jpg
What is the "Ausnehmung" or "recess" in the slide for?

 

[Foxy]

I believe the ausnehmung is the groove where the slide lock locks into the slide (least, that is what the groove in the slide on my pistol does )