In fact, and according to Spanish monarchists, beginning with the late Count of Barcelona, don Juan Carlos was Prince of Asturias from 1941 to his accession in 1975. The wedding invitations listed Juan Carlos as such.
However, Juan Carlos and Sophie used the title Prince and Princess of Spain from 1969 onward.
Arturo Beeche
(always a supporter of don Juan until his resignation as Head of House...although I was delighted with the monarchy being restored on Juan Carlos, who has done a great job as King!).