Window balancer (metal Milgard window)

Thanks for posting. Your window balance reminds me of the S380-21 or the S385-21. To determine which of these is the correct replacement, you'll need to confirm the depth (front to back measurement) of the bottom shoe.

You'll likely need the same 2040 stamp. A pair of these balances will support a sash weighing 15 to 23lbs.

Comparison of the S378, S380 and S385 balance shoes.