Igros Moshe, Even HaEzer Chelek 4, Siman 60.
That teshuvah is not directly discussing mixed seating (he is discussing “having a girlfriend”).
I am pretty sure I remember a teshuvah discussing mixed seating at weddings. I will, bli neder, look it up tonight and find the exact teshuvah.
I don’t know what to make of their testimony. I would venture a guess that R’Moshe’s opinion is a chiddush and that he himself didn’t arrive at this conclusion until sometime in the sixties.