This is just the kind of abject nonsense that I have come to expect from generative AI assistants. For example, just a few weeks ago while researching a set of drawing instruments, my search for the owner’s name along with the word “architect” prompted google’s AI to invent a non-existent architect of the same name, along with several public buildings that he had[n’t] designed.
However, there is a case to be made for fine lead predating the 1960s. As I was recently surprised to discover, the “H” leads of the 1820s were essentially the same size as the modern 0.7 mm, and there was an even smaller (albeit little-used) “VH” size that was presumably closer to 0.5 mm, all long before polymers were even a glint in the industrial chemist’s eye.
By the time of my metal lead gauge, VH had fallen out of use, but I will try to measure one of the leads when I get the chance.