In "A Life on the Road" I believe he mentioned he was using Augustines but carried additional sets with him due to his frustration with G string manufacturing tolerances/issues. This was in the early eighties.
1973 Ryoji Matsuoka No. 30 Cedar 650mm
1976 Masaru Matano 600 Cedar 658mm
2015 Francisco Navarro Concert Cedar 630mm
1972 Yamaha GC-6D Ezo Spruce 662mm
ISTR that in the movie "My Life In Music" he didn't mention any particular label, but did say that he carried a micometer with him to check the diameter of strings. I don't think that strings alone give him his extraordinary tone, but also the Hauser model guitars, fingernail density and so on. Pretty sure he could pick up a cheap guitar and make it sound great.