I think they are captive nuts as opposed to cage nuts. I found some on a website here but you had best be sitting down:
http://www.thelongolivedrabline.com/...-nut-14-20-nc/
http://www.thelongolivedrabline.com/...c-front-floor/
I tried looking at the online spaenaur catalogue, but I hate online catalogues. I had an old hard copy of their catalogue at one time...wish I knew who I gave it to.