I emailed Peggy Seeger and asked her about the two unissued albums. She was able to supply the list of songs that she sang on them:

ZDA 76    Argo       12â€쳌          THE LONG HARVEST
          (unreleased)          Record 11 - Ballads Resident and Migrant
                         1.    Henry Lee
                         2.      Fair Annie
                         3.      Half-Hitch
                         4.      Matty Grove

ZRG 3029/    Argo       12"    THE LONG HARVEST   
ZDA 77    (unreleased)    Record 12 - Second Crop   
                         1.      The Ballad of Springhill
                            2.      Omi Wise
                            3.      Tyburn Hill
                              4.      Hiram Hubbard
                         5.    Floyd Collins
                         6.    Lolly Tudum
                         7.    Wicked Polly
                         8.      John J. Curtis

She also suggested that the Long Harvest might be issued on CD, but recorded from vinyl rather than tape as the current owners of the tapes (Polygram) say they are lost. That's a great pity, but at least a re-issue from vinyl would be good for people who can't do the same themselves. Plus they would, I hope, be proper CDs not CD-Rs. (Some of the first CR-Rs I made for myself years ago have now stopped playing on my hi-fi, although they still work in my car and on PCs. (So you just burn another!))