Thanks for all your replies. Bruce Kís description is pretty much spot on. My boat is from round about 1970ish and Iím assuming it was original fit.
The lever on the right operates a flap-type valve, directly through a very short vertical pipe through the hull
. No other piping involved.
Method of operation is...
1. With lid closed, briefly lift
lever on right to let a little water
2. Lift lid and do necessary
3. Close and clip down lid
4. Lift lever and swiftly push down dome, ensuring small hole in black top handle is covered.
5. Release lever and pull dome up.
Really simple and would be easy to clear if it ever blocked. Only disadvantage is itís not suitable for inshore, harbours, anchorages
Wondering how Ďsafeí it is and whether I should replace it.