Listen to those flats. Did they even have roundwounds back then?
The Rotosound Swing Bass 66s made their debut in 1966. They didn't start regularly showing up in music stores until '69-70 and gradually started getting more popular over time. It was pretty much Entwistle and Squire who put them on the map and popularized them in the US. And they really took off once the original progrock world adopted a Ric and roundwounds as that genre's signature bass sound. So figure around '72-74 before they really started getting popular here.