Commonly found in older cars. 50's - 60's 70's for attaching tonneau covers, boots, some convertible tops also found  on some sports cars and foreign cars. 
10-32 machine thread. Diameter of Snap Head is 3/8" across. Stud is 3/8" long, comes with locking nuts.
This does not include the top snaps, (the ones on your cover).
If you need the top snaps... (caps and sockets),  let me know I can combine a kit for you.
Thanks!