Yeah, The Embury Theory. I’ve got an inkling the change takes place a decade later, when Browne Vintners takes over US distribution and convinces Noilly to produce the rouge. Most formula changes are driven by importers. Browne launched the sweet red in 1955-56 and I’ve always thought that it’d make sense if they had changed the dry at the same time. But there’s a distinct lack of evidence.
The Noilly Prat archive is currently (or was?) being worked on at Bénédictine. I was trying to organize a visit before… well, before. Hopefully, the answer lies somewhere in there.