More information for the non technical inspired by the following comment from my brother...
Actually, I'm not. I lied.
This is such a JM sentence: "The key spins the toggle at the top and pushes down on the cam directly below it, which then contacts the plates and interrupts the natural motion of the gear selector when the lock is engaged."
My non-technical, non-mechanical head is spinning!"
Lets break it down. It aint too hard.
"The key spins the toggle at the top and pushes down on the cam directly below it, which then contacts the plates and interrupts the natural motion of the gear selector when the lock is engaged."
This is the shift lock not installed yet in the bus, and with the cover plate off. The cover plate acts as the housing for the keyed lock , which would be located on the upper portion of the mechanism.
On the backside of the key mechanism is a square peg which catches in the channel on the key toggle, such that when you turn the key, you turn the square which turns the channel which turns the toggle.
Turning the toggle pushes down on the cam which rides on a pivot on the left. The bulb on the lower right of the cam pushes down on the shift plates and holds them in place...
...and when the plates are locked in place, the natural motion of the shifter through the 'H' pattern of the gears is interrupted. If the lock is open and the cam is not pressed against the plates, then the shifter is available for use and the plates simply slide around on top of each other, free to rotate on their own pivot.
Clear as mud? Good! Any more neophyte questions and the cat gets it.