Hey, I can't find this info, either on Mavic's website or at my LBS that has the rim. Does anyone know?
The reason I'm asking is that I had a wheel with an MA40, that cracked at one of the eyelets, I think because it had been built with too high spoke tension. As such, the spokes probably don't have much fatigue and I'm looking for a rim with similar E.R.D. (effective rim diameter) to rebuild the wheel with the same spokes. MA40 has 613 E.R.D.; one option for rebuild is Sun M13II, which has an E.R.D. of 610mm. The Mavic MA3 had an E.R.D. of 611mm, so if the Open Sport is the same (the O.S. is hte replacement for the MA3) it should work fine. The Open Pro has a deeper cross-section of 605mm that probably wouldn't work with these spokes.
For the record, ERD is measured to the end of the spoke, usually flush with the head of the nipple.