Milgard window balance

Hello there. If the metal channel, not including the nylon attachments at both ends, is 21" long, then this might be either the S380-21 or S385-21. The only difference between these two balances is the 'depth' of the bottom nylon shoe, or the 'heel-to-toe' measurement. See the diagram below for reference. Measure your old balance's shoe to see what matches best. Let us know if you have further questions.

Comparison of the S378, S380 and S385 balance shoes.