I think the shape of the guitar would usually be a clue as to whether it's steel strung or nylon strung. I guess it's nylon strung if it's a small-bodied Spanish guitar. I've heard a recording of Elton Hayes playing the owl and the pussycat recorded in 1953, don't know about Burl Ives.

Does anyone have any recollection of guitarists playing in sessions or with fiddles, flutes etc as opposed to singers? I'm wondering whether the guitar existed in that role at all before the folk revival. Or even through the folk revival, as that seems to have been largely based around singers rather than tune players.