Stroud Reliance Mk2 or MK3 And EcoPure 40mm Multi-Port Valve to Filter Flange O-Ring
The 40mm valve was fitted to 21″ 25″ and 28″ With the 28″ some had the 50mm valve, They were attached to the tank with bolts where the 40mm is held by a band.
There were two Reliance filters and valve Pre 2006 MK11 Part Number is M8101 (155mm diameter) post 2006 Part Number is M8101PET MK111 (180mm diameter 5mm thick).
This O-Ring may require replacing when you service the filter and change the sand, If it starts to leak in normal running you may require a sand change, As the filter is building up too much pressure.