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Lauren from SWISCO responded:
February 21, 2024
Hello, thank you for posting, It determines you have either our Series 351, with a 15-002 bottom attachment, or Series 390, with a 15-001 bottom attachment. Take a look at the image below and measure to determine which balance you'll need. The length of your metal channel excluding the nylon attachments should measure 15" long. The 1490 stamp that indicates the strength of the balance spring does not directly translate. You will need to obtain the weight of the window sash and compare to the strength chart on the product page to choose the correct stamp option. Please review and let me know if you have any additional questions. 
Comparison of the S351 and S390 balance shoes.
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