dood plate ID question

[Walter White]

my brother-in-law showed me the hexagon shaped pop rivets holding his plate on yesterday and told me they are hard to get. I had never heard that before, so I rushed home to look at mine. I was relieved to see the rivets don't appear to have ever been replaced on mine.

That's good to know as Ive questioned my 72 truck's history because of the rear view mirror mounted to windshield frame.

What else do I not know about those door plate pop rivets? Thanks