Board Thread:Naruto Discussions/@comment-3568954-20140530034624/@comment-1512913-20140606191450

The 3 tomoe are hanging on the ring, so there would be no tomoe without the ring.

I insist my theory to be correct, unless the manga proves me wrong of course ^_ It's only logical. The original Rinnegan (eye of Shinju and third forehead eye of Kaguya) was only 1. As Kaguya gave birth to Hagoromo, he inherited said eye power in both his regular eye sockets pair, but it got split in half. So rather than inheriting the 9T Sharinnegan/original Rinnegan in each, he got a pair of 3T Sharingan and a pair of tomoe-less Rinnegan.