kingkegcap

4" Black Plastic Beer Barrel Cap with Valve

If it's made of black plastic then it's probably this one.

Barrels that it won't fit are, Hambleton Bard, Saffron, Boots or the old barrel that lies on it's side

Complete with valves

Choice of S30 or Pin Valve, look at the description of the valve on this page to work out which you want!