I'm thinking it's a type of collar nut although they all seem have internal rather than external threads.
FYI - This nut is for the Bendix version of the 1968 Sapphire IX radio available as a dealer-installed option on VWs and the only radio I know of that uses them. There is also a Motorola Sapphire IX version which is internally quite different using a standard thin nut that attaches over the shafts to hold the faceplate. I was contacted by someone to see if I had any left and could only find one in the radio donor pile.