I just added this quote to the Alto Trombone History Timeline. Special thanks to Joost Geevers, Conservatory of The Hague.
1968—New York: In a rehearsal of Beethoven’s Leonore Overture No. 3 with the American Symphony, Leopold Stokowski says the following to the principal trombonist: “Do you have an alto trombone? Do you have one? Could we use it in this? I think it has another quality of tone. Alright?” (YouTube excerpt of rehearsal filmed in 1968 at Madison Square Gardens concert hall; the excerpt is later used in a 1970 documentary titled, “Stokowski at 88”).