Casey from SWISCO responded:
September 28, 2020
Hello, It looks like you have either our S351-29, with a 15-002 bottom attachment, or S390-29, with a 15-001 bottom attachment. Take a look at the image below to determine which balance you'll need. You'll also need to weigh your window sash if there is no stamp on the metal balance channel to determine the correct spring strength option. Thank you! Comparison of the S351 and S390 balance shoes.
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