He chose Margaret Price, who was almost completely unknown, and even fought for her when the producers objected. They surely had plenty of Barbarinas to pass around at the time. She had already sung Cheribino somewhere, I believe. (That role used to be almost exclusively sung by sopranos until the mezzos decided it was theirs.) Klemperer obviously had an ear. I can’t recall the actual key signature but Act III at the wedding ends in the major and Act IV opens in the minor of the same key. When they’re played back to back, it’s delicious. A gem.