To celebrate the 25th anniversary of the Tony Awards (and to de-emphaisze the fact that the 1970-71 Broadway season was a fairly weak one, overall), 'producers Alexander H. Cohen' and Hildy Parks came up with the inspired idea of saluting the 25 previous Best Musical winners ("Fiorello" and "The Sound of Music" having tied in 1960) by having musical numbers from each show performed that night, many of them with the original cast members. Thus, audiences at the awards and on TV got to see once-in-a-lifetime performances by Zero Mostel, Gwen Verdon, Alfred Drake, 'Robert Preston', Yul Brynner, Carol Channing, and many other legendary performers, some of whom rarely appeared on TV otherwise, making this the Tony award show musical theater buffs still talk about almost four decades later.
It wasn't until more than three decades after this special aired, that it was revealed that a very few performances that night were lip-synced.
This entire ceremony is available to view free of charge on YouTube.