Casey from SWISCO responded:
October 28, 2020
Hello, The stamp number you've provided does not match up to our spring strength options but if you are certain that the S350-28 is a match to your balance, you can weigh your window sash and follow the chart below to choose the correct option. Thank you! 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 |