Achenbach windows use a small wedge/distance holder to ensure the correct position of the window when it gets closed. The original part from Achenbach is basically only a shell and tends to break after a couple of years (depending on the heat/UV exposure + amount of window usage).

It can be printed rather easily but needs to be post-processed. The angled wall and the bottom wall need to be sanded down to be smooth. Otherwise, there will be some resistance and grinding between this part and the window frame when it is closed.

This part is easy to replace on normal windows but might not be easily accessible on french windows. For the latter, the window might need to be remove completely from the frame to reach the part on the bottom. Never do this alone and only under supervision of a professional.