Casey from SWISCO responded:
July 17, 2020
Hello, It looks like you have either our S351-30, with a 15-002 bottom attachment, or S390-30, with a 15-001 bottom attachment. Take a look at the image below to determine which balance you'll need. If these balances we're removed from the same window, the stamp number should have been the same. It's possible that someone replaced one of the balances with what they had on hand, which means one side of the window had more tension than the other. I would recommend weighing your window sash and following the chart under the product description on the balance product page to make sure you choose the correct spring strength option for your replacement balances. Thank you! Comparison of the S351 and S390 balance shoes.
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