I believe we all remember report where Ali suppose to say that concealment of Mahdi would last Six days or six months or six years
Here we have another interesting date from “Irshad” of shaykh Mufid.
Hadith 11: From Thalaba ibn Maymun from Shuayb al-Hadade from Salih ibn Maysam: I heard Abu Jafar (alaihi salam) said: “Between rising of Qaim (alaihi salam) and murder of Nafsu Zakiyah not more than 15 days”.
This hadith is very clear evidence that shias of time when these 3 narrators lived didn’t knew that there gonna be 6 more imams before al-Qaim would be even born! They wouldn’t dare to fabricate such hadith if idea of 12 imams and concealment of 12-th of them would be famous and known among shias.