Gravity is a cruel mistress, and garage floors are basically black holes for small parts.
If yours decided to go on a permanent vacation during a maintenance session, or you just realized your T865/T880/Norte is missing it, here is the factory replacement.
 
It’s the exact same M9 flap that came with your tablet. No magic, no fancy upgrades—just the piece you lost so you can get the convenience to screw/unscrew your cable from the back of the device.