Window Channel balance springs

Thanks for posting. Is the metal channel 28" not including the nylon attachments at both ends? If so, this looks like either an S380-28 or S385-28. The only difference between these two balances is the depth of the bottom nylon shoe, or the 'heel-to-toe' measurement. See the image below for reference. Compare both balances with your old hardware to see what matches best.

Comparison of the S380 and S385 balance shoes.

You'll also have to choose a stamp option, which reflects the strength of the balance. Look for a stamp on the metal channel that indicates this. If there is no stamp or you are unsure, weigh the window sash that these channel balances support. You can then match that exact weight with one of the weight ranges listed alongside each stamp option under the product description of each and every channel balance shop page.