The OD is 44mm, and the lip seen at 36mm should be a tangent radius. The cross-section, seen here:
is a 4mm line (the flat) tangent to a 4mm radius arc which is then tangent to a 30° angle. The tangent arc doesn't print, it turns into a 0.5mm step, which is unacceptable, needing post-processing to sand it out and remove it.
Have printed about a dozen examples with them all being the same.