The AP cages are cut braces of PVC bolted to the back of the cage wall with 2 screws each. They have a little lip on the front which the shelf just sits so it basically wedges the shelf between the lip of each brace and the back wall. Also the back wall of the cage has channels routed into where the braces fit so the weight isnt put on the screws, its actually put on the spot where the bottom of the brace sits in the routed channel. Here are some pics that might help.
They arent glued or anything so you can just lift and slide them out of the cage when cleaning.
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