In his review https://www.musicwebinternational.com/2023/01/rossini-il-barbiere-di-siviglia-pristine-audio/ Mark S. Zimmer asserts that Callas had never sung Rosina on stage before she took part in this EMI recording. As a matter of fact she sang the role at La Scala the previous year with Gobbi and Alva (as in the EMI) with Giulini conducting. The production was a flop but it was recorded, possibly at the premiere in February 1956 and there exists a CD-issue (see review: http://www.musicweb-international.com/classrev/2008/mar08/Rossini_Barber_PHO8015.htm The sound is lousy and the singing leaves a lot to be wished, but it shows that she didn't go the Kingsway Hall unprepared.