You’re looking at a 3-gallon “cornie” keg which we use to serve cask ale (alongside a gravity cask too). Before the pitchforks and torches come out (cask enthusiasts like their tradition!) let is be known this is a concept we picked up on while visiting England.
We “cask” condition our beer in the cornie exactly the same as we do in a traditional cask. We then hookup the cornie to a hand pump and use a “breather” to fill the headspace with CO2 caused by the draw from the pump. This preserves the beer (not allowing any spoiling O2 to enter) but doesn’t carbonate the beer whatsoever. Basically this is a bit of the old and a bit of the new – as mentioned it’s a practice now used throughout the UK. – Rulpsen