This is for my coupe listed in my signature, a 68 Sprint B. I did notice this thread is for the fastback but my search for the 377522 nut located this thread.
On page N8-8500-3 page 36 I believe of the body assembly manual for the coupe pictured on the left of the page, it's item C listed under number 3 and is 377522-s100 and is called a crown nut. The page has both the coupe and fastback pictured. Maybe I'm reading it wrong but the nuts on my car are flimsy nuts that appear to be light metal and are not solid.