The manufacturer says this button has a 7/16" shank. The actual shank diameter of this button is 0.406"

3/8" = 0.375"
This button = 0.406"
7/16" = 0.4375"

For mounting sheet metal and fiberglass, quarter turn panel fasteners are the most practical fasteners requiring only one tool. Panel fasteners have been used in off-road racing for years due to their lightweight and durable design. Available in a variety of sizes to fit your specific needs.

Originally quarter turn fasteners came from the Military Aircraft industry. When they say #5 they are referring to the diameter of the button which is 5/16. Or if they say #6 they are referring to the diameter of the button which is 3/8. The length of the button is measured from the panel surface down to the bottom edge of the spring seat.