Number on old is 21 12 and number on new is 2120
Bought the 2120 as figured it was stronger than the 2112.
Tech installed, and they didnt keep the windows open. Not even close. The spring on the old one is 3 inches longer and considerably thicker.
Please advise...
George
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Casey from SWISCO responded:
May 2, 2023
Hello, As long as the 1-1/8" measurement below matches the depth of your bottom fitting, I agree that the S390-22 would be the correct replacement. Your 21 12 stamp is a bit strange and, as you mentioned, does not correspond to any of our strength options. For this reason, you would need to weigh your window sash to determine which strength option you'll need. There is a chart on the product page that explains how to choose your option based on the sash weight. Also, please note that the length of the spring does not correspond to the amount of weight that it will support. Thank you! 
Comparison of the S351 and S390 balance shoes.
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