This plastic piece is attached to the top of a Milgard single hung aluminum window. It's the stop that holds the balancer in place. The aluminum windows were manufactured in 1997. This photo is of a complete, intact stop. I need to replace it on some other windows where it is broken.
I couldn't find a section of your website to identify them. If you don't carry them, maybe you know where I might get them.
Sincerely,
David