I'll look for a pic, but I thought those were influenced by oil pressure more than centrifugal movement. Still done the same way on modern engines, but they have solenoids to help control the oil flow.
Well, I looked it up quickly and found this thread:
http://www.jalopyjournal.com/forum/s...d.php?t=629885
Clearly, I was wrong. I'm pretty sure I've seen someone with an old oil pressure variable cam.