I have their song "Dubuque" from the multi-artist Young Fogies (1985) by Dr. Humbead's band; the recording, according to the liner notes, was from 1970, but the group at that point did not include Jim Bamford, who was apparently replaced by Eric Thompson on guitar in that recording.

Apparently, Bamford retired from playing with the NTSB by November 1970 due to ill health (source), though Banjo Newsletter of Feb. 2020 (click) posited that Bamford's leaving had "something to do with the presence of the female member."

Besides that, I haven't found much. I found this page (click) which writes the following: "Mac Benford and his band left and headed east. Jim Bamford, guitar player with Dr. Humbead, died."

Based off what I can find on digitized newspapers, Benford went east around 1974 (though mentions of his name dropped off entirely from 1971-1973), suggesting that some time between 1970 and 1974 Bamford passed away.

But, I can't find anything for sure.