Pretty obvious that there was a reason why they waited to revived the hokages. It could only be done in the shrine because the tablet reacts to the reaper mask. The stone was likely left by Rikudou to his two sons, but as a result of the division/the creation of the juubi, we know it didn't work out that way. As oro begins the hokage's unsealing he puts the mask on and crouches down at the tablet. No coincidence i'm sure. Theres more to the tablet then meets the eye. I'm not sure though because there was no suggestion that sasuke read it with ems. He only showed regular ms when he met the 1st.