I think some years used two different sized retainers. The ones on my 73 were the same size. One bracket was black and the other was plated though. The bracket that is larger goes to the rear. On mine, two brackets were opened up more then the other two. I put those to the rear since they hold the buckle.
For some reason, I couldn't get the sharp locating tab to line up correctly. When the tab was in its hole, the bracket was all turned weird. The brackets under the headliner had never been removed so I don't know what the problem was. I tried swapping the brackets front to rear and side to side but they just wouldn't work with those tabs inplace. I just ended up removing the tabs on all four of the brackets.