Peter -
You're probably thinking of the Mavic 300 series headset which did have an alloy pinchbolt, so it absolutely could not vibrate loose. It was not to stabilize the stem however.
You can see a pic at
http://www.bikepro.com/
Find them occaisionally on Eboy for 35-50 bucks, american.
Mark Petry
Bainbridge Island, WA