need to identify channel balance

A home owner from 84098 says:
I need to replace several channel balances. The existing balances are from a 3'x4' single hung metal frame window installed in 1984. I also need to replace balances from a 4'x 4' single hung metal frame window installed on the same date. The balance for the 3'x4' window is stamped 2340X and the 4'x4' window is stamped 2340. All balances are 24 inches long.
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Lauren from SWISCO responded:

Hello, thank you for posting! 

Based on your pictures, it looks like you have either our S351-24 , with a 15-002 bottom attachment, or S390-24 , 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.
Comparison of the S351 and S390 balance shoes.
You will need a 24" balance with stamp option 2340, meaning your window sash weighs between 18-23 pounds. Please let me know if you have any questions. 
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