This is the plastic piece that fits on the end of the window extension. Just the top part that slides into the window is broken. The bottom piece has the number 31879 on it.
Tom from SWISCO responded:
March 22, 2022
Hello! We sell both pieces you show in these photos: see our 39-558 detach clip and 39-536 slide.
I think these are just what you were looking for but please let me know if you needed anything else! I'm always here to help.