Window balance replacement

Hi there. If just the metal channel, not including the end attachments, is 21" long, then this is either an S380-21 or S385-21. The only difference between these two channel balances is the 'depth' of the bottom nylon shoe. See below. Measure your old balance's shoe depth to see what matches best. For a stamp option, choose 2040, which will support a window sash in the weight range of 15 through 23 pounds.

Comparison of the S378, S380 and S385 balance shoes.