So how does the original A/C Center Vent work?

[Orange '71 GMC]

How is it held in place? I got my duct piece, gasket (which is just a piece of very thin foam), and the center vent, but I don't get it.

The center vent has to holes in the back, but they aren't threaded. Then the duct piece has a set of holes that line up in the back of it, but how does this go together?

Pardon the idiocy...I haven't been frequenting the board long/much, and there are some things I didn't learn back when I was doing my original build up.

Thanks in advance guys.