Those are called "sex bolts".
It looks like from
this link that they originally may have used semi-tubular rivets. That would be the easiest type to put them back together with. You can get those in various sizes from
Hanson Rivets in LA. They also sell setting tools for them.
It looks like finding the reflectors that go in the arm will be harder to find than rivets. Hope you can get it restored. I remember those on school busses and trucks when I was a kid.
