Found on SO SO MANY GM USA pre metric (aka around mid 80's) GM body construction.
Bolt has a pointed end is 3/4" (21mm) and has an incorporated washer head.. not loose.
These bolts & cage nuts were used on secure say inner fenders.. many brackets & stays.. anywhere a nut was to be secured over an opening for the bolt to go through.
10 bolts 5/16 - 18 x 3/4
10 Cage nuts. 5/16 - 18
10 bolts 3/8" x 3/4
10 Cage nuts. 3/8" - 18