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Casey from SWISCO responded:
November 11, 2019
Hello, It looks like you have either our S351-28, with a 15-002 bottom attachment, or S390-28, with a 15-001 bottom attachment. Take a look at the image below to determine which balance you'll need. 
Comparison of the S351 and S390 balance shoes.
It appears that you have a 2790 spring strength stamp, which is a weight system that is no longer being used. To determine the stamp options that you need, follow the chart below based on the weight of your windows bottom sash. Stamp Option Sash Weight Range 2710 5 to 9 lbs 2720 9 to 13 lbs 2730 13 to 19 lbs 2740 19 to 26 lbs 2750 26 to 30 lbs 2760 30 to 33 lbs 2765 33 to 36 lbs 2770 36 to 39 lbs |
