disston pretty well nailed it.
I will add that the small slotted machine screws thread into brass inserts in the housing. They often corrode and seize in the brass. When removing those screws, if they don't unthread easily - stop! Continuing often breaks the insert loose in the boss, breaking the plastic.
The best action when they're stuck, is applying some penetrating oil. Work the screw a fraction of a turn each way to help the lube penetrate down in there.
On re-assembly put a dab of anti-seize on the threads to help prevent future problems.
If the lenses get bumped it's possible for their mountings to come unglued. With the eyebrow removed, gently pull up on the lens. If movement is detected, it'll need resealing.
Wanted: Dead, smashed, crashed or trashed gauges
BMW GAUGE REPAIRS - TACH*SPEEDO*CLOCK*VOLT METER *PODs & LIGHT BOARD*