The square of metal you refer to are the clips which should be pop riveted to the body, or it may be the bolt head referred to below.
On my 76 there is one bolt/nut at the forward end of the trim piece. The "slider" which forms the head of the "bolt" actually fits into the end of the track on the back of the trim and can move laterally to help align the piece on the car. The nut for this is located on the back of the fender in a narrow trough. It is a bit of a pain to remove, but not impossible. The clips are designed so that installation (after the nut is loosely tightened) is by hooking the top edge of the track on the clip and applying pressure to the bottom edge. All being well the piece will snap into place,one clip at a time. Then you can fully tighten the nut.
To remove these trim pieces you should first back off the forward end nut, then pry gently on the bottom of the piece. This will spring the bottom track from the clips and allow you to lift the piece off.
This may be TMI, but I found out all of this stuff the hard way myself.
Regards
Al /ubbthreads/images/graemlins/canpatriot.GIF