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Emily from SWISCO responded:
November 30, 2023
Hi there! Thanks for reaching out to us. I agree that you have either the Series 351 or Series 390 but it's hard to eyeball the size of the bottom shoe from a photo. You would need to measure the depth as shown below to determine the proper Series. There should also be a stamp on the metal channel but it may be too rusted to make out. If that's the case, you'll have to weigh the window sash on a scale and refer to the chart under the product description on the S351-28 and S390-28 store pages to determine the accordant strength option. Let us know if you have any questions! 
Comparison of the S351 and S390 balance shoes.
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