Balboa 40mm Heater Gaskets
Balboa 40mm Heater Union Gasket
O'ring has one side with a slightly rounded edge and the otherside a thicker square edge.
O'ring is approximately 58mm in diameter, 6mm in total thickness(includng the rounded and rectangular edges) and 2.5mm not including the raised edges.
This is commonly used on the GS100 10 heater tube.