21" non-tilt window channel balance

Thanks for posting. This looks either an S380-21 or S385-21 Channel Balance. 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 hardware and see what matches best. For a stamp option choose 2020.

Comparison of the S378, S380 and S385 balance shoes.