As it certainly isn't a Rinne-Sharingan like Madara's and Kaguya's, and since it has tomoe, could it be a mixture of two Rinnegans in one eye? Hagoromo gave the power of his eyes to Sasuke, which would imply that he gave Sasuke the power of BOTH of his Rinnegan. Perhaps Sasuke's single eye possesses the power of both of Hagoromo's eyes?