77569 is the fat o-ring, material about 0.270 inch. My engine is an IO-360-A1A, in a 1967 Mooney M20E. It requires the skinny o-ring, Lycoming part 74712, material about 0.148 inch. They are both made to seal the two inch pipe in the induction housing assembly, but there’s two different intake pipes. The counter guys at Spruce (West) are very helpful.