Window spring

Thanks for writing in. I agree, this looks a lot like the S385-34! The only other possibility is the S380-34. The lone difference between these two balances is the 'depth' of the bottom nylon shoe. If you haven't checked this already, compare both balances with your old hardware to see what matches best. 

You'll need to select a stamp option. It looks like there's a stamp code on your metal channel, but I can't quite read it in your photo. If you're uncertain what to choose, weigh your window sash. You can then match that exact weight with one of the weight ranges listed alongside each stamp option on the product page.