Snape and Mad-eye Moody also know it's Riddle since they poison his father's grave. Minerva was present when Dumbledore said that, and did not comment on it ('surely not the heroic Riddle, our old ally!'). So we have multiple examples, but they're all trusted key members of the Order who even know about Horcruxes, so they don't tell us anything about when the identity was found (assuming I'm right).