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Bob from SWISCO responded:
June 2, 2023
Thanks for writing in. If the metal channel, not including the nylon fittings at either end, is 21" long, then you have either our S351-21 or S390-21. The only difference between these two balances is the depth of the bottom nylon shoe, or the 'heel-to-toe' measurement. See the image below for reference. Measure the depth of your balance's shoe to determine which Series matches what you need. 
Comparison of the S351 and S390 balance shoes.
If the window weighs just over 13 pounds, then choose a stamp option of 2030. This will support a window sash in the weight range of 12 through 15 pounds. Let me know if you have any other questions. |
