Block and tackle window balance

Thanks for writing in. This looks like either an S380-26 or S385-26 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 to see what matches best. For a stamp option choose 2550.

Comparison of the S378, S380 and S385 balance shoes.