What is the extra switch for,  leds?
Also what makes these 20th's different from a ,lets say Mark King signature which would be 32 same electronics I think and even the same body shape and omega and Laminates

I don't know about the Annivesary, but the standard scale for the MK is 34.  
Signature electronics have a p/up pan knob, while Anniversary electronics have a p/up selector switch & add a stereo/mono switch for the ? output.  And while every Anniversary I've seen a picture of has the Large Standard Omega body, the MK starts with Large Standard Point; the omega is an option.  There may other differences as well - anyone?
 
Peter

The 20th Anniversary guitar and bass have unique shapes. They are smaller than the Standard Point/Omega style, but they are not the 3/4 shape.  
 
The woods on the 20th Anniversary were Walnut top and back with a Walnut core and Vermilion accent laminates or Quilted Maple top and back with a Maple core and Purpleheart accent laminates.  
 
Anniversary basses were in 32 and 34 scale.  
 
They also have the 20th Anniversary jewelry on the peghead.

Here is a thread of interest with pictures on this topic. Such a beautiful Bass, I had the good fortune of playing one once in a Southern California music store in 2009.
http://club.alembicguitars.net/Images/411/22886.html?1133311684