Giulio Mauri never existed. Both the mandarin and the zingaro are sung by Nicola Zaccaria himself!
This little secret had been revealed by Nicola Zaccaria himself some years before his death (2007). Although we 're not sure about the reasons, rumour has it that the singer who was to sing the Mandarin was not good and that Zaccaria agreed to sing it himself with the condition that the other singer would get fully paid.
Giulio Mauri was just created by Zaccaria to pay tribute to Giulio Neri while he was recording Il Trovatore with Karajan and then re-used it for Turandot.
John Ardoin in his book The Callas Legacy also mentions Giulio Mauri as a pseudonym of Zaccaria.