Balboa/Gecko 50mm heater union O'ring Gasket.
Suits most Balboa 50mm Controllers & M7 heaters.
Best replaced in pairs
O'ring has one side with a slightly rounded edge and the otherside a thicker square edge.
O'ring is approximately 74mm in diameter, 6mm in total thickness(includng the rounded and rectangular edges) and 2.5mm not including the raised edges.