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Casey from SWISCO responded:
August 17, 2021
Hello, I agree that it looks like you have either our S351-21, with a 15-002 bottom attachment, or S390-21, with a 15-001 bottom attachment. Unfortunately, we can't confirm dimensions from photos but take a look at the image below to determine which balance you'll need. Your 20 D stamp would normally indicate that you need a 2040 spring strength option for your replacement balances but if your window sash is exactly 15 Ibs., either the 2030 or 2040 strength will work for you. See the chart under the balance product description that shows the complete breakdown of the available strength options Thank you! 
Comparison of the S351 and S390 balance shoes.
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