Hi SWICCO,
We need at least 4 of these plastic shelf bracketts to hold up a second shelf in our Greenhouse Window, installed in our home in the early 1980's ('82-84, i think. You have the picture of the brackett next to the ruler attached. The round/cylinder shaped end fits into a track of plastic round holes in each corner of the greenhouse window. The rectangular (with curved corners) which is before the round end, locks this brackett into the track with plastic round holes in each corner that holds up the wire/steel shelf of the greenhouse window.
Thanks, Joe