My sense is that the word "retro" was not in common parlance until the Eighties
Certainly when researching Shock + Awe, I didn't find it once in the contemporaneous writings about Roxy Music, although later on the idea that they collided future and retro in deliciously entangled ways would become a critical commonplace
But here's an early use of it from Peter York, in his famous piece on "Them" (a clique / milieu of style hyper-conscious provocateurs, an avant-garde of sensibility) from Harpers & Queen, October 1976
"The Sex people hate Retro, and seem perfectly sincere about it"
(Sex meaning the people in and around McLaren and Westwood's boutique - and the quote seems to be defining the Sex / punk people against Them's irony and pomo traits - ie. "they mean it, maaan", they're in deadly earnest)
Seems likely the first use of "retro" would be in fashion and clothing retail and / or the graphic design world