Spring loaded piston for keeping upper sash up.
A quick learner says:
June 14, 2016
This part keeps upper sash up. There is one on each side of the sash, the one on the right sash came out of its sleeve and will not go back in.
Paul from SWISCO responded:
June 16, 2016
The part you're looking for is known as a balance. Yours in particular is a spiral balance. I would recommend looking at our Series 600 to start out, just make sure that the diameter of your outer tube is 5/8" (9/16" should be fine, too).