Tony Rees 07 Nov 18 - 01:27 PM pointed to 'a link marked e.g. "20003_FS0661_0001.mp3" '. If you do "save link as" you get a file with that name which however is not an mp3 file at all but a page of HTML.

Playing the audio does work, so you could capture it as it plays.

However the page on which the link appears does say "Patrons must secure permission from donor before personal copies can be made. Copyright is retained by the authors of these recordings, or their descendants, as stipulated by the United States copyright law." Dunno how you find who to ask for D.K. Wilgus Papers.