Norman Blake's song, Church Street Blues, has the line, "I wish I had some guitar strings, that old Black Diamond brand. I'd string up this old Martin box and go and join some band."
I could buy Black Diamond strings in the early sixties at Milligan's Grocery Store in Hamilton, Ontario. They gave you extra unwound strings.
Mapes used to also do this. I think I've been given a set of Mapes in the last ten years with extra unwound strings, and if I remember correctly they gave me my choice o a wound or unwound banjo G(3rd) string.