Stephen Schwartz, co-author of Spanish Marxism versus Soviet Communism: A History of the P.O.U.M. in the Spanish Civil War, has linked a number of his associates to the film below (which is available on Youtube). Your editor, in turn, passed the link on to another distinguished author of a recent book on the Spanish Civil War, Adam Hochschild, who was wowed: “Amazing. I’ve seen lots of still photos from this time—which must have been in the first month or two after the beginning of Franco’s coup—but didn’t know there was film.”
https://www.youtube.com/watch?v=UVv6ampdPeY