I recently bought a set of lock tumblers and re-keyed the locks on system panniers and the topbox mount on my K1200GT.
The tumbler kit came with this diagram for the ignition barrel . . . .
From the diagram you can see that there are eight tumblers (four on top, four below), plus an unused slot.
When the correct key is inserted, all tumblers are retracted against their springs, allowing the barrel to turn. The squared end then turns the lock striker to 'unlock'.
All that is holding the lock barrel in is the small sprung plunger at the end opposite the key.
There is a small hole in the switch mounting that allows a straightened paper clip, or similar, to be inserted to depress the plunger. once depressed, the lock barrel comes out with the key still inserted.
I can't remember whether the plunger aligns with the small hole when in the locked or unlocked position (presumably 'unlocked' but the lock wouldn't come out without all tumblers depressed anyway).
Caution . . When replacing the lock, make sure the plunger faces the hole, not 180 degrees out or you will have a real job on your hands next time (DAMHIK

).
And . . If any tumblers fall out when you have the lock out, make sure that when you replace them, all tumblers are FULLY retracted when the key is inserted BEFORE YOU REPLACE THE LOCK, or it won't work and won't come out ('cos its now got the wrong key !).
Hope this helps.
Bob.