A home owner from Beaumont, Texas says:
April 5, 2014
This is a channel balance from an aluminum window, Alenco circa 1985. Has marking of 27 C. Measures 28 inches. I have identified the top piece as 19-002 but have not identified the bottom piece, or at least one that is close and might work. From looking at other customers post, possibly the S390? Hoping you can advise. Thanks so much.
Tom from SWISCO responded:
April 8, 2014
Hey, thanks for posting. It could be the Series 390 balance, yes, but you might want to confirm that it's not the Series 351 instead.
Now, as you can see, there is only one difference between these two Series, and that is their bottom shoe. The Series 351 uses the 15-002 while the Series 390 uses the 15-001. I can't quite tell what you have based on your pictures, though. Please carefully compare their dimensions to your own hardware and see which is a better match.