Thanks guys. I probably haven't explained it properly though.. I can't see any extra holes on the on/off, it has a big nut/screw/disc of some sort right in the middle, the same size and in the same place as the purge knob, only it has a recess for an alan key and looks like it will come out easily.
The Purge one must come apart (for instance if I wanted it anno'd) but I can't get into it to unscrew it. I suggested plyers on the inside nut but Lump said no, lol.
Is there someone who does anno jobs that would know how to get it off? Then I'll be able to see if it fits the other one? Thank you