Channel balance replacement

A home owner from Highland Falls, N.Y. says:
They measure 25' metal to metal.The only stamp is 2420.Do you think i need one for each side? 2 windows are involved. Thanks
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Casey from SWISCO responded:

Hello,

It looks like you have either our S351-25, with a 15-002 bottom attachment, or S390-25, with a 15-001 bottom attachment. Take a look at the image below to determine which balance you'll need. You'll also need 2420 spring strength options for your replacement balances. 

When replacing balances, we always recommend replacing both sides since if one breaks, the other normally isn't far behind and the extra tension of replacing one balance will generally cause it to break. Thanks for posting!

Comparison of the S351 and S390 balance shoes.
Comparison of the S351 and S390 balance shoes.
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