I've just got a new Bushnell Elite 4200 6-24x40 and can't make out how the adjustments work.

The manual shows a 'bar' type adjuster which can be removed by removing two small screws with a jewellers' screwdriver and set to give a returnable zero. It also shows the calibrations as 1/4" per click (some are 1/8").

What I have is different. It has a (apparently non removable) drum with a knurled ring at the top and, intriguingly, another knurled ring on top of that, as seen here that doesn't want to turn on its own. The calibration size isn't given:

http://www.straightshooters.com/bush...1650turret.jpg

How does this system work?