Thank you OP.
Mr. Niven recorded these from vinyl LP's to cassette tape, presumably using the technology available in 1975-1982. The 30+ year old cassette tapes were played analog into a desktop, and software converted the files to MP3 at a bitrate of from 96 to 160.
Just for grins, a couple years back I converted my old car stereo tapes to MP3, and the results weren't even close to what I could get from playing the LP's directly into the desktop.
The LP's are indeed treasures - but do you want to listen to your dad's old cassette tapes from the '80's?