Limbo. He used Limbo pre-Juubi Jin on the Bijuus and we seen all of Madara's Rinnegan jutsus from Nagato who mastered the Rinnegan well. I think he had Limbo in both eyes like how Shisui had KA in both of his MS. This also explains why Nagato and Obito couldn't use it, neither one could access Madara's MS/EMS jutsus at all or we would be seeing Susanoo from Nagato. This is before becoming the Juubi Jin so it's no powered up Rinnegan powers either. It's not confirmed or anything so this is speculations but more things points to this being true than false.