Window balance

Thanks for posting. It looks like this might be either an S351-29 or S390-29 Channel Balance. 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 depth to see what matches best. For a stamp option choose 2840 in either case.

Comparison of the S351 and S390 balance shoes.