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Emily from SWISCO responded:
June 27, 2022
Hi there! It appears you have either our S351-21 or S390-21 channel balance. These two parts look very similar at first glance but the important different is the depth of the bottom shoe. The Series 351 comes with a 1" deep shoe and the Series 390 comes with a 1-1/8" deep shoe. You'll need to measure as shown below to see which one matches best. The stamp on your balance is outdated and doesn't match any of our options. With that said, you'll need to weigh your sash on a scale and refer to the chart under the product description on the S351 and S390 store pages to determine the appropriate spring strength option for your balance. 
Comparison of the S351 and S390 balance shoes.
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