Commonly found in older cars. 50's, 60's, 70's, & some 80's for attaching tonneau covers, boots, convertible tops also found on some sports cars and foreign cars. 
8-32 machine thread. 3/8" long includes Locking nylon nuts.This does not include the top snaps.
If you need the top snap caps, or nuts, or handheld snap tool.....let me know I can combine a kit for you.
Thanks!