Well, if your 72 is set up like my 73, the casting is held to the shell by Pop rivets that are installed from the inside side of the skin. If you are very lucky, there will be a plastic access cover on the inside that might give you access to the rivets as well as the backside of the wiring going into the housing. I am assuming you haven't removed all your interior skins.
The little stubs you see are probably the broken off pop rivets--you will need to carefully drill them out of the casting, which will leave the hole that the new pop rivet will go into.
If you want to reattach them without having access to the inside of the skin, pretty much your only option will be to drill through the casting from the outside, and make a new hole in the shell, then put a pop rivet through everything to hold it in place. If you put the rivet in the blue "groove" around the casting, you can paint the rivets blue and they won't be so obvious.