Window balance

Thanks for posting. This looks like either the S380-29 or S385-29 Channel Balance, depending on the depth of the bottom shoe. See below. Measure your old balance' shoe depth to see what matches best.

Comparison of the S378, S380 and S385 balance shoes.

You then have to choose a stamp option, which reflects the strength of the balance. Your old balance's stamp is from a rating system we do not use. That '40' stamp though suggests the sash weighs 40 pounds. Therefore choose stamp 2870, which denotes a balance that can support a sash in the weight range of 39 through 42 pounds.