Window channel balances

If the metal channel length, not including the nylon fittings, is 21", then this is either an S380-21 or S385-21, depending on the depth of the bottom nylon shoe. You'll have to pick a stamp option. If there's no stamp code on the metal channel, obtain the exact weight of your window sash. You can then match that exact weight with one of the weight ranges listed alongside each stamp option on the product page.