While we are on about Callas rarities/possibles/might
be/rumours/legends, according to the author of the Callas
book "Sacred Monster", there exists a copy of one of the Callas
performances of Don Carlo at La Scala in April 1954. One of the five
performances was supposed to have been broadcast but I guess we
would have heard about it had it still been in the RAI archives. I
suppose we will just have to wait until one of these Fafners
disgorge their golden hoards. There is always hope - look what has
emerged in recent years from Buenos Aires, and these were fabled
recordings that we thought probably no longer existed. Here is the
cast of the Callas Don Carlo from the Frank Hamilton Performance
Annals (an invaluable resource - thanks again Frank)

Elisabetta di Valois Maria Callas
Don Carlo Mario Ortica
Rodrigo Enzo Mascherini
Filippo II Nicola Rossi Lemeni
Il grande inquisitore Marco Stefanoni
La principessa Eboli Ebe Stignani
Un frate Antonio Zerbini
Tebaldo Sandra Ballinari
Il conte di Lerma Luciano della Pergola
Un araldo reale Giuseppe Zampieri
Una voce dal cielo Greta Rapisardi