Mystery rubber piece on my Agattu, where is it supposed to go?

I taken the plastic covers off the motor drive unit on my 2010 Agattu as I wanted to swap out the sprocket on the motor, I took 3 pieces of plastic off, when in hindsight I probably only needed to remove the plastic covering the sprockets.

Anyway, after I removed the 3 covers, I noticed a small piece of rubber on the floor, and was wondering where it came from and where it's supposed to go upon reassembly?

THanks