99% sure it was Fields but not 100% sure because of that 'last year' thing.
If Jim sees a musical common point, that is very strong evidence. I trust his ears.
Two more angles to pursue:
1) This is not the full concert. The announcement at the end of CD2 corresponds to the first set since he says 'we will be back'. If the second set announcement exists, it will mention the guest. This is exactly why these tapes should be in an archive for researchers and not at the mercy of record producers to chop up and resequence.
2) Fowler told me the Stitt/Thompson was recorded but he did not know where the tape was. If it ever surfaces, we will know if Fields is on it. Give me two hours in that basement...