Zero frets were common on guitars and basses made in Europe from the 1950's and beyond. Many of the European makers such as Hopf, Arnold Hoyer and Olympia Musima, used them, often on quite expensive jazz style arch tops and of course Hofner also used zero frets. In the USA, Gretch and Rickenbacker also used zero frets. So even before Japanese mass production in the 60's and 70's zero frets were in common use. The first instruments I saw with zero frets were back in the early 70's, they were on a mandolin and a arch top guitar, both European made, from the 40's or 50's.