Window balance help to select the correct part

A quick learner from Waukesha, WI says:
May 9, 2023
This window balance if for a non-tilt out window. The markings on the side say 26 4 BQ CALDWELL. The length of the actual metal portion of the balance is 27" and it's 1/2" wide. From the ones available on your site it looks like it could be an S351 or S390 but I can't tell which one. Thanks for your help.
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Jasmine from SWISCO responded:
May 10, 2023

Thanks for writing in! You are correct, your balances do appear to be either the S351-27 with a 1" shoe or the S390-27 with a 1-1/8" shoe. To determine which series is needed for your window. Look over the dimensions from the diagram below to confirm the depth of your bottom shoe. You'll need to select the 2640 stamp.

Comparison of the S351 and S390 balance shoes.
Comparison of the S351 and S390 balance shoes.
27" Series 351 Channel Balance
27" Series 351 Channel Balance
S351-27
$10.77
27" Series 390 Channel Balance
27" Series 390 Channel Balance
S390-27
$10.55
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