These are the rivets. You place them in a hole and use a small punch in the middle to expand the "tabs".
Phillips head screws is an interesting subject. They were not all that popular back in the early days. They became more popular when electric/pneumatic drivers were used on the assembly line. The driver was self centering and self locating. OK for low torque applications. If you see Phillips head used on the plate and nowhere else on the truck................red flag.
This post is bouncing between how to tell if a plate is original by the looking at the fasteners................ and how to attach a plate........... and how to make it not look suspicious. Let us not forget that you are not supposed to attach a plate yourself without DMV supervision. The Stovebolt is not here to help otherwise. There is no
legit reason for a plate to have ever come off, even to sandblast or paint the truck. If it's off, call the DMV.
You can't buy a title and plate and use them.
You can't buy a blank plate, stamp it and use it.
You can't take a plate off of a salvage truck and use it.
You can't do anything without DMV instructions and inspection.
Getting away with it is no proof that it is OK.