Hm ? Wasn't Naruto's KCM and Bijuu mode explained to be different because it wasn't the Kyuubi's chakra alone, but one mixed together with Naruto's ? From the current revelations we could link it up with Ashura, but then it would mean that Minato also had at least a portion of his soul, hmmmm (The Ashura's soul standing behind Naruto's back does give out a flame aura, so maybe in KCM/BM it emanates outwards ??).
Hmmmm, Naruto entered KCM right after taking the Kyuubi's chakra and only then used the sealing jutsu to imprison the Kyuubi's mind, to which Kurama remarks to it being So6p's...
The next thing we know, Kushina, Mito and Karin were also reincarnations of Kaguya (with portions of her soul), thus their particular special strong chakra suited for dominating the Bijuu... funny how we moved from everything depends on the genetics, to everything depends on the transmigration of the soul .
For Mito to remain as the first Kyuubi Jinchuriki, Ashura's Cloak would need to be his own, or from the Jyuubi part of Bro6p (though this would imply that Indra's Susanoo is bigger than the Jyuubi).
So, looks like the real source of Uzumaki symbol, is a closed circle , because when it's open, it's a black hole/dark sun.