Replacing sash balance

Hey Ken. This looks like a Series 770 Channel Balance. It's a bit tough to tell, but if the metal channel is 28" long, then you need the S770-28. You also have to choose a stamp option, which designates the balance's strength. Look for a stamp on the metal channel that designates this. If there is none, you can use the exact weight of the window sash the balances support to determine which stamp option to pick. Let me know if you have further questions.