Need replacement for Caldwell channel balance

First, I want to confirm something. Are your "20 4" balance and "20 5" balance in the same window? Each window sash should have the same balance on the left and right sides. If your stamps are different between two balances holding the same sash, you need to weigh the sash and choose the proper balance for that weight.

That aside, you need to buy a 21" balance with a 2040 stamp option for a "20 4" stamp, or 2050 for a "20 5" stamp. Verify this by weighing the sash, as I mentioned. You'll then need to choose your balance Series. This looks like either the Series 380 or the Series 385, but I can't tell without measuring. I need you to measure your bottom plastic shoe as shown in the photo below. If, as depicted, it's a 1-3/32" shoe, choose S385-21. If it only measures 1", you're better off with S380-21.