PSA had the hot pants first. When Southwest was beginning their operation, they came to PSA to see how they did it and bring back as many ideas that worked back to Texas. (Remember at the time, PSA flew only in California as an intrastate, Southwest was doing a similar thing in Texas.) PSA provide the "Love Airline" with an older version of their uniforms that were not in use any longer.
Someone else mentioned the Braniff strip. My dad flew back from Yakota AFB
in Japan to Travis AFB
in California in late 1967 on s Braniff charter to the military. He said the flight attendants changed outfits at least 7 times during the flight (I believe it was 13 hours in a DC-8).