No. 2: (36mm Diameter) Birch & Lime
Number two Keystones are used to seal the faucet bushing and facilitate tapping (broaching) of the beer by the publican.
They can be supplied plain made from birch or lime, or bored and plugged (birch only) with a plastic Tut in red or blue.
The bored and plugged variety allows for the addition of finings with the cask stood up, the re-sealing being affected by inserting another Tut.