Wikipedia gives a year of 1981, but there's no source (which I'm trying to fix unless I give up on that article as well.)

I've looked through both "King of the Godfathers" and "Five Families" and found nothing. In fact, the latter (p. 637) references he was elected in a manner that suggests he never officially got the blessing of that position.

I think he got some sort of position after the Napolitano murder.

i thought it was after rastelli`s death

I'm not sure when he became underboss. I know he was made in 1977, promoted to capo in 79, and became boss in 91

Hit up Google News. Apparently between 1986:

http://www.nytimes.com/1986/04/17/nyregion/motion-for-secret-jury-in-rackets-trial-denied.html

And 1987:

http://www.nytimes.com/1987/08/27/nyregi...ed=3&src=pm

He got bumped up from "reputed captain" to "reputed underboss". Unfortunately there isn't much from after everyone turned states.

why not send the fat rat a letter in prison and ask him im sure you`ll make his day

It'd be interesting trying to get it passed through WP:RS

I think Massino was never formally made underboss. He got to be first among capos by staying on Rastelli's good side, by avoiding drug-trafficking at a time when the Bonannos were drawing too much attention, and especially by outmaneuvering his fellow capos. It's not unheard of in Mafia families for someone who was not officially counted as underboss, or who ranked below an underboss, to become the Don.