Thank you for your helpful video on how to identify a part! I can't get a picture of it installed because it's hidden on the side of the sliding aluminum window.
It's a channel balance (I think that's what it's called).
24 3 GL Caldwell
25 inches long (metal length only)
Bob from SWISCO responded:
April 5, 2021
See our S351-25 or S390-25. The only difference between these two balances is the depth of the bottom nylon shoe, or the 'heel-to-toe' measurement, so compare both with your old hardware to see what matches best. For a new stamp option, use 2430.