The complete assembly including the piece that welds into the vessel is $95.00 +GST from us if anybody is interested - we stock a few bits and pieces like this for our micro customers. The whole inner assembly is removable for cleaning and the actual valve and seal is a single piece of food grade silicone rubber with a very light spring which holds the valve on its seat until there is enough pressure to force past the seal after the tap is opened. The clamping mechanism that holds the assembly together is Triclover in a 1/2" size.
I meant to put Zwickel's name in the topic header as I thought he may be interested in this particular Zwickel.
Wes