My GUESS is that the hole is a tell-tale, as seen on on classic car water pumps- it basically lets oil flow out when the pump seals are worn, so you know when the coolant pump is likely to fail. Lynx Engineering would know for sure- I usually speak to John Sandys.
Sure there is a gauz or filter behind the disc.
Screw a bolt into the threaded part and slide hammer the disc out once the circlip is removed , clean the filter and fit new o ring to the disc.
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