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Hello there!

Can you confirm that the outer tube measures 3/8" in diameter? When determining the diameter, you want to measure the outer metal tube, not the spring itself. If it is 3/8", see the Series 900. 

It looks like your old balance has a yellow tip which is rated for a sash weighing 24 to 34 pounds, not 18.56 pounds. If your window has been working fine all this time, you could just stick with the original yellow tip. You can also weigh your sash on a common bathroom scale to confirm.