Plastic Piece for my window

I am looking for a plastic piece for my window and have no idea what it is called or who makes it. The window is over 20 years old and any manufacturer title isn't shown. What they do is they support the wibdow by being attached to a string that is attached to a spring while having a hole to hold up the window through a rod. We need the piece because as soon as the window is unlocked the top window instantly falls.