“Installation hole” provided in the header, see for example : http://farm4.static.flickr.com/3645/3673135595_01cec5895e.jpg
I have changed this valve due to oil leakage a few years ago. The new seal is a rubber (Viton) O-ring instead of the original metal seal that becomes flattened and causes the leak.
These pictures are just for archive. No problem with this currently.