Leafield A6, the white dot version. List of all availables valves is here
http://www.leafieldmarine.co.uk/Docs...A6%20Valve.pdf
Fitting involves cutting a hole in the tubes and glueing a hypalon circular patch onto the tubes.
They can be positioned wherever you want on the tubes, conventionally they are ususally fitted horizontally in line with the inflation tubes on the rear chambers 10-12" away. The one in the bow directly opposite the inflation valve