Although Leo didn't invent the electric bass, he made the first commercially available one. He came up with the 34" scale. Gibson came out with their electric bass the EB a year and a half later and decided on a 301/2" scale. They were both trying to come up with an instrument so guitar players could double easily and get more work. Leo also put foam under the bridge cover which coupled with the flat wound strings mimicked the tone (thump) of the upright bass. Most players back then did play with their thumb so a finger rest was logical for the new instrument.
Even by the mid sixties, the electric bass was still in it's infancy, as far as instruments, strings, amps and playing styles. A lot of bassists just sounded like whale farts to my young ears.