Gwern and others who have dug into it this far might be interested by this footnote in the Crespo article: "I have tried to lay my hands on the original version of the conversation, as I am sure Simon did, too. I contacted Gabriel Zadunaisky, who, as the article explains, participated in the meeting. He is a professional translator. I asked him for the original version, and he replied on WhatsApp: 'Mr. Crespo: I am very sick. Unfortunately, I am unable to provide you with the information requested.' My hypothesis is that Zadunaisky translated the conversation directly from the recorded version and that this original version has been lost."

My read is that most likely, it was recorded on an old school reel-to-reel tape recorder. It's entirely possible that the tapes are still sitting on a shelf somewhere in Argentina, though the chances of actually tracking them down are pretty low. I worked with some reel-to-reel tapes that Alan Ginsberg made (now held at Stanford) in the mid-60s (including one where he is talking to Bob Dylan!) and they held up pretty well. Had to use audio editing software to remove tape hiss, but they were not as badly preserved as I expected.