TargetTalk

Where the weights pivot out of the front of the action, there is a 'crescent moon' part kind of dangling. There is no mention of it in the manual.
It appears to do nothing.

What the hell is it?

Are you talking about the low-pressure indicator at the front of the trigger guard? When the gun is pressurized, it gets pushed 'forward', basically flush with the trigger guard. If the cylinder pressure drops below some level (I haven't measured when that is exactly), it starts to creep rearward. The idea is that you'll notice that before your shots start dropping off.

On mine, I never remember to look (or feel) for it, so the one time I've had it function, I didn't notice until my shots were already affected. However, I do use it as an indicator that the gun is depressurized when I unscrew the cylinder.

marky-d

You have a PM.

MarkyD nailed it. It is a generally worthless low-pressure indicator.