Buster Ardito implied on a bug that they opened the books in 1974. George Barone says early 70s. Gravano said they opened the books in 1975, but didn't induct anyone until early 1976. I think that's what Massino means too. In Funzi Tieri's FBI file, an informant says two people were inducted around February in 1976.
I think opening in 75 and inducting in 76 makes the most sense.