We are way off on a tangent talking Ric basses and the history of stainless roundwound bass strings but...
According to this article - The RotoSound Swing Bass Strings came out and were available for sale in 1962.
The Birth of the Roundwound Bass String
"In 1962 they brought out the first roundwound bass strings; they called them the 'Swing Bass' set, and the sound of the electric bass changed.
Entwistle was soon a convert to the new roundwound bass strings, but after using them for a few years he thought that they could be developed further.
He contacted James Howe at the newly re-christened 'Rotosound' string company and helped to develop a new set of strings."
"British bass players loved the brighter, more defined sound that these strings made, and it helped create the sound of the 'beat groups' such as The Kinks and The Who, and later bands like Pink Floyd, Yes, Black Sabbath and Led Zeppelin."