Hi: My theory is that "Lygia" wasn't earmarked as a single when Mario recorded it, but that shortly afterwards either he or someone associated with that recording session (e.g. Sam Weiler, Ray Sinatra) urged RCA to release it. Of course, it's equally possible that RCA simply wanted to save a few bucks by having Mario record it at a Coke Show session and then make a decision afterwards as to whether it was singleworthy or not.
Incidentally, it's interesting that at the same session Mario also recorded "Begin the Beguine," but that rendition was rejected. He would go to re-record it a month later. Thank God he didn't settle for a substandard take, as so often happened on those shows.
I have no idea what year the two Chenier arias (along with two Rigoletto arias) were released as an EP (Mario Lanza in Opera) in the US, but I"ve always assumed from the cover that it was during the 1950s. According to this information, the EP was first released in Italy in 1957, so I assume that it came out in the US around the same time if not before:
But I would imagine that the flip side of the original 78rpm released in the US of "Un di' all'azzurro spazio" was the other Chenier aria: "Come un bel di' di maggio." (That 78rpm was mentioned in glowing terms in a 1967 review of the LP ML Sings His Favorite Arias.) Of course, for almost five decades the "Come un bel di'" was always---inexplicably---the rarest of all Mario's RCA operatic recordings, as it never appeared on any LP release in the US or the UK (only RCA Italiana released it, as far as I know, and that was many years after Mario's death). Mind you, the great Improvviso recording was almost as rare! Two examples of idiocy on the part of RCA.
Cheers,
Derek