I fitted one of those enfield duck bill valves to the standard breather tube on my series A Comet. It works great. Remember though the breather on a series A is just an open pipe from the rocker area of the cylinder head and is not a timed breather.
Does it just discharge into the air or do you have a way to capture any oil mist that comes out? I bought one but haven't tried it yet on my C Rapide.