Anchor hole insert for spiral balance

Small square hole in balance channel near the top of window should have a plastic insert that grips the screw that secures the spiral balance. Plastic insert sits in the square hole and flush without protrusion. My other windows have the insert. Didn't think to take a picture of it while I had spiral balance out (duh). Hole is the width of a number 2 pencil or probably a size 8 screw. I can't install a spiral balance until I have an anchor for the balance.