Herman miller shell: Shock mount question  

these are not shockmounts, ju...
these are not shockmounts, just washers. they are usually not glued to the shell. the threads are built into the shell so you would at least need a seat-pad to cover them if you remove the upholstery.

is it often mounted this way...
is it often mounted this way when the shell is upholstered?

Yes
The later upholstered shells were made this way... probably starting sometime in the 1970s.

did anyone try to take of...
did anyone try to take of the upholstery? If yes, I'd love to see a picture of what it looks like.

not mine
but that's what it looks like.
http://www.flickr.com/photos/thbonamici/2082880234/

Some or all of them
Are glued down, so you would need to do some stripping and refinishing, if you want to remove the upholstery.

" some stripping"
it's quite a drag, really.

I was ready to spend the...
I was ready to spend the time for that. Now I'm worried. Did anyone sit on one that has the screws? do you feel it?
Otherwise I could put on some real shock mounts and glue them with white epoxy for the white chair and black for the other and then sand what come out on the other side. It would make a smooth surface again...

Lotta work
Not much reward. Even if you are able to yank, tug, strip, and pull all the glued foam off you're still gonna have two chairs with 4 holes in the bottome of each one. Possibly more holes, as the upholstered ones often had air holes drilled.
For this type of chair you're almost better to just wait for the right shell to appear. There are still plenty of shells out there!
I found an upholstered side shell in the trash and tried to strip it. After 25 minutes it went back in the trash for another poor soul with a better work ethic than me 😉