Actually, I was in a trio in 1969 called Speed Limited, running flats on my P bass. Rounds had been invented, but I hadn't heard about them. I'm not sure if Noel Redding used flats with Hendrix or Mel Schacher with Grand Funk, but it sounds likely. I think John Paul Jones used flats with Zep and Jack Bruce did with Cream. Dee Murray used flats with EJ.
Probably 75% of all the classic Rock, Country and R&B recordings were made on a P-Bass with Flats. That is THE sound.