Mom Jeans
I watched the film this week and it was funny to see how much Brando's and Lee Marvin's jeans are what would be called today "Mom Jeans". When I saw the movie for the first time in the early '80s the style seemed perfectly normal since everybody wore high waisted pants.
My father, who was twenty years old in 1954, told me that in Southern California the style at the time was Levis 501s that were never washed from new, engineer boots, and big cuffs at the ankles. That's pretty close to what the BRMC and Beetles wore.