Can you confirm the stamp and length of the balance? If the stamp is 26-5, that usually indicates a length of 27" long. Are you sure the stamp doesn't read 16-5 instead? That would make more sense with a 17" long balance.
If you are confident that the length is 17" and that the stamp reads 26-5, then I recommend weighing your sash on a scale to get its weight. That is the only other way to figure out what you need. There is a chart on the
S765-17 store page that shows you how much weight each option can support. Once you know how heavy your sash is, you can simply refer to this chart.
