I found it here in a 1956 Allied catalog. Take a look at figures E and K. Although made for twin-lead, they fit a standard FT-243 socket. https://worldradiohistory.com/hd2/IDX-CATALOGS/IDX/Allied-Catalogs/Allied-Catalog-1956-OCR-Page-0233.pdf I’ll still make one from a FT-243 holder and maybe one day one of the clear plugs will find me. Paul, W9AC