Milgard window balance

Thanks for posting, and for all the thorough information. This looks like either an S380-21 or S385-21. The only difference between these two balances is the depth of the bottom nylon shoe, or the measurement from 'heel' to 'toe'. See the diagram below for reference. Compare both with your old balance to see what matches best. For a stamp option choose 2040.

Comparison of the S378, S380 and S385 balance shoes.