When did the Chinese central government first officially confirm the 56 nationalities?

Actually, it was in 1979 that China officially confirmed the 56 nationalities.

Fiona is not quite correct. During the years of the 'Republic of China,' the government recognized five nationalities (Han plus four minorities -- Manchu, Mongol, Tibetan, Muslim). Under the PRC, the initial field investigations that led to formal state recognition of 'minority nationality' status were conducted in the early 1950s. Subsequently, some groups were added to this list individually -- the most recent addition coming in the late 1970s. In addition, there are other groups that claim a distinct ethno-national identity, but have not received official state recognition.