window balance.

Thanks for writing in. This looks like either an S351-19 or S390-19 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 1850.

Comparison of the S351 and S390 balance shoes.