Gut strings were used for music, surgery and tennis.
Music strings were the most difficult to make: many different gauges, and a high quality standard.
Managing orders with so many different products and dealing with musicians requests... it was not an easy task! While surgery and tennis were bigger markets and more safe, they also they required a slightly different string, with different characteristic.
So, during and after WWI, these two productions mixed up, being more convenient for string makers to follow the bigger markets. String makers were usually not involved at all with music or musicians, they were (small) business owners, concerned more of raising their kids than supporting arts.
Modern harp developed technically using these modern kind of strings. But early music instruments lost the vocal tone they had until the 19th century. Advertising focused on new problems: perfect fifths, even strings... but the durability of the string became a serious issue: that's why, as soon as they could, musicians switched to modern materials, which came available only after WWII.
The years between the two wars were really tormented, also from a music technical point of view. It’s when we had the major changes in playing technique.
Video from Bow Brand:
