Greetings,

Your question is not so clear, do you mean which university's Ph.D. is applicable for assistant professorship in old IIT's?

The simple answer to that is none as there is no such preference given to particular universities as  long as the applying candidate shows merit and experience in his/her field. Given that there are many professors at old IIT's who have done Ph.D. from IITs themselves.

Hope it helped, if your question remains unanswered then do rephrase it and post again. thanks