I should note at this point that this should be done indoors over a table where you can find small parts. There is supposed to be a small steel ball that sits on the small spring next to the lock cylinder (just below the cylinder in the above picture), that helps keep the flap over the key entry in either the open or closed position. I took the cap apart outside standing next to the bike and never saw or heard the part drop. I only noticed when I put everything back together and the flap was loose, and I searched the site and found out about this part.
The gas cap separated into an upper and lower half. The upper half contained the tumbler, the lower half contains the two catches that hook a lip inside the tank.
Inside were two loose parts. After some tinkering, I figured out what the parts were and where they went:
The two bumps on the bottom of the lock cylinder engaged the slides that retract the catches on the lower half of the gas cap. The loose parts appeared to be part of the vent mechanism. Depending on how the gas cap is oriented, the steel ball can roll blocking or unblocking vent holes.
Because the metal cap holding the steel ball in place came loose, it fell into the path of one of the catches, preventing it from fully retracting.
I put the ball back in the hole and pressed the metal cap back in place:
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