I have Lincoln Window Casement Screens for which many of the plastic corner keys have broken. These are pre-1999 screen frames with the corners cut on a 45 degree angle. These older keys are no longer available from Lincoln. I was looking at your website and it looks like the Andersen 3/4" x 7/16" keys might work or be able to make work.
Do you know if this is the case? if not is there another key that you have, like Pella, that might work?
Roger