Thanks for writing in! To confirm if the Series 160 Coil Balance are the balance springs you need for your window, I'll need to see a photo of the coil assembly. You'll also need to confirm the width of the coil.
To determine the spring strength needed for the coil with the missing stamp, you'll need to obtain the weight of the sash the coils are supporting. Unfortunately, there is no way to determine if they are the same without the stamp. Once you have the sash weight, you'll divide it by the number of coils in the window.
Please let me know if you have any questions.
Since the sash weighs 19lbs, based on the formula mentioned, you'll need the S140-10. This coil balance only has (1) coil on the assembly, however it will work just the same.
Please let me know if you have any other questions.
