I guess "little-known" or "well-known" is a bit subjective, as each of us has no real way to tell how many others knew about these people back then. Likewise, I may never have heard of somebody listed here who was known to many others, who would appear to be "well-known" even though I've never heard of them.

It could be argued that nearly all folk singers then and today, are largely little-known by people outside the folk community, even some of the biggest names. How many people outside the folk world have heard of (say) John Kirkpatrick, Jez Lowe or Vin Garbutt? (Just to pick three names more or less at random, of highly respected folk artists who have been around a long time and are big names in the folk world, at least in the UK).

That said, I have heard of some of those listed by Frank Hamilton - Derroll Adams, Mike Bloomfield and Hedy West I would have thought were fairly well known, but maybe that's just because I know them. As I said, it's subjective.