Engineers and supervisors obviously had to gain some experience with stereo mixing. Rudy Van Gelder wrote in his notes to the SACD issue of Coltrane's album with Johnny Hartman (which included both stereo and mono mixes) that engineers were told to do the mono mix with special care until the late 1960's. Considering the fact that stereo turntables took many years to dominate the market, this is only logical
I once had an old Teleton receiver with a blend knob from stereo to mono. A priceless feature.