The retainer cover - sometimes referred to as the input shaft cover - mates to the front of the transmission inside the bell housing and is where the throw-out bearing sits. The clutch fork then slides the bearing on the snout of this cover, which then applies pressure to the pressure plate diaphragm, engaging the clutch. It is a very important and often overlooked piece.
NOTE: In accordance with various minor changes with the mid-cycle refresh, this air filter will only fit automatic versions of 2017+ BRZs and Toyota 86s.