That much is possible - Fender Mastersound Flatwounds did appear in 1959 - but not a given.
This is the internet forum vulgate, but it is incorrect.
Going backwards:
- John Entwistle famously developed and endorsed the Rotosound Swing Bass set in late '66, but he did so because
- he'd used a Danelectro Longhorn and fallen in love with it and the Black Diamond roundwound strings it came stock with (possibly ever since its introduction in 1958 and earlier with the UB-2 six-string bass), also available in shops as replacement sets at least in part of the US; and
- because James How &Co. had actually been making stainless steel roundwounds since 1962; they were probably hard to find even in the UK, except in nylon tapewound version (How initially made them for Burns of London bass models around '62-'63, and the sets were also sold in shops under that marque, before the 1966 introduction of Rotosound-branded Tru Bass black nylons)
- what about other basses from the Fifties? Rickenbacker had a deal with German company Maxima (now Optima) that supplied them with nickel flatwounds; ditto for Höfner, with their Pyramid-made flats; of early Kay or Gibson bass strings I regrettably know zilch.
As regards pre-1959 Fender, you're in for a surprise: the sole bass-guitar string set they marketed back then, the No. 90 Pure Nickel,
-- has been declared to be as "#notflatwound" by Fender historian Terry Foster
-- was still made twenty+ years later, and at that point simply described as "roundwound" by Ernie Ball in their 1977 string gauge guide, listing their own offerings compared to those of the then-current competition
-- was recently acquired, in its early CBS ('66-) incarnation, by TB member @TwototheValley and shown, with photographic proof, to be
He further adds
- By the way, pre-Fender electric bass guitars by Paul Turmarc (1936) and his son Bud (late 1940s) also wore roundwounds.
