Window balance

This appears to be either an S380-35 or S385-35. The only difference between these two balances is the depth of the bottom nylon shoe, or heel-to-toe measurement, so compare both with your old hardware to see what matches best. For a new stamp option choose 3450. 

The stamp option designates the weight range the balance is calibrated to support. The old balances had a stamp code of 34-5. This conforms to our stamp option of 3450, which denotes a balance that can support a window sash in the weight range of 30 through 34 pounds.