Larger pics:
2007:
http://finemodelcars.com/hi-res/hi_res_ ... 0%2001.jpg
2008:
http://www.finemodelcars.com/hi-res/hi_ ... l%2008.JPG
A to H = Various different settings for diff and fuel and such.
E will be for Traction Control as the colour for the TC button is the same as this rotary, TC was operated as 12 being most intrucive and 1 being least intrucive. 1 was pre-set as 10% intrucive and 12 being as 100% on, so 12 settings, take that as 7.5% increments of how much TC a driver wanted.
G is fuel, as McLaren standard practice was for Yellow Golf to be set to setting 4 for the in lap.
The numbered rotary is live engine mapping. As back then drivers could change maps on the fly, this stopped recently with the off throttle blown diffuser tech, many drivers liked this rotary, however now it is gone they have to drive arround it, Mimi and Lewis liked to keep it set arround 5 as it gave a good performance for their driving style, Alonso and Montota liked to keep changing that one on the fly as they constantly wanted different torque demands. Its ironic that McLaren put the numbered rotary on a thumb wheel for 2008 as Alonso requested.
I cant find a 2005-2006 wheel, but id suggest thjay a Y button was for a confirm button if a driver cant get on the radio. like "Felipe Fernando is faster than you, can you confirm, thank you" thing where he wouldnt want to talk to anyone and would hit that button.