Casey from SWISCO responded:
March 20, 2023
Hello, It looks like you have either our S351-27, with a 15-002 bottom attachment, or S390-27, with a 15-001 bottom attachment. Take a look at the image below to determine which balance you'll need. There should be a stamp code on your metal balance channel that you will need to choose the corresponding spring strength option for your replacement balances. There is a chart on the product page that explains how to choose your option based on the sash weight. Thank you! Comparison of the S351 and S390 balance shoes.
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