newshound has brought this to us :
Often not sufficiently captive though. They seem to be centre punched to
secure them; presumably glasses shops have a neat jig for holding them
securely, with either a press or a guided punch. I have often thought about
asking them how they do it.

The worst struggle, was getting the screw started. I tried starting it
with the lens in place and I couldn't get the frame gap closed up
enough. I then finally did it by just starting the screw, then
squeezing the lens in, before screwing it in all the way.

My plan c, would have been to pull the frame together with the lens in
place, by using 5amp fuse wire around the brackets, to enable the screw
to start.