Pack of 6 Nitrile O-rings which are slightly larger than the stock green G&G O rings (Highlighted in images 2 and 3).
These are intended to be replacements, but they also slightly improve air seal, specifically the air seal between the Gas Reservoir and the Gearbox on the gearbox-side silver connector piece.
I go over these in my upgrade video here.
Dimensions are 6.5mm OD 4.5mm ID 1mm C/S