Mystery solved.
Note the "lip" that extends from the bore of the 80 mm cylinder at the top. It centers the head gasket and sets the cylinder length for the engine geometry. The difference between the height of the gasket surface and the lip is 3 mm. The cylinder length to the gasket sealing surface is around 82.5 mm. The length to the top of the "lip" is around 85.5 mm just like all the other 911 cylinders that do not use a thick head gasket.
