It was but from 1964 on the French Pro was held in the Stade Pierre de Coubertin on indoor wood. Between 1930 and 1963 it was held in Roland Garros (thus on clay) but once in 1953.
As I said I think Laver was at his best in 1967 but I can't really say. I've not read history books about tennis. Others can tell you better than me.