Buick rotor orientation

Just got a replacement rotor for my 364nailhead, the underside appears to have one small square post, and one round one, does anybody know if this has to mount a certain way round on the distributor, or can it fit either way? I recently took the old rotor off while instaling new points and condenser, but didn't notice any kind of distinguishing alignment holes in the top of the distributor where the rotor mounts. Just some strange plates with springs, but both sides looked the same.

 

Those are lining up the rotor.