Aligning oar blades for a sliding seat system

I am building a pair of "hatchet" type oars for a double racing Shell.  I have a pair of Dreher oars and they are very nice and priced accordingly.  I'd like to replicate their action particularly in alignment of the blade both during the "drive" and during the recovery.  The Drehers have 3 flat spots on the collar that align with flat spots on the oar lock.  During the drive the flats force the blade to be nearly perpendicular to the surface of the water and hold it there and during the recovery the feathered blade is angled slightly up to reduce the tendency to "catch a crab."

 It appears that the flats on the oars are not all perdendicular to each other but I am not able to measure the angles accurately.  It is possible that they are actually perpendicular and that the control of the angle is accomplished by the flats on the oar locks.  Does anyone understand the geometry involved here and can help me get this right? 

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