Need Caldwell Block and Tackle Balances

A handy person from Dunedin, FL says:
My approx. 44"x 60" window, single hung , vertical slide, tilt-to-clean window: the block and tackle balance came unstrung on one side(broken string). I removed the other side which still looks quite serviceable. Shoes still look good. It is a Caldwell 26 9 K04S CMC.

Also, I know the balances are probably fifteen years old. Should I go ahead and replace both balances?
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Casey from SWISCO responded:


Your balance resembles our S765-27 channel balance. The stamp indicates that you may need a 26-9 strength option but you can also weigh your window sash to confirm that it falls between the 30 to 34 lbs range supported by the 26-9 option.  

We always recommend replacing both balances at once, especially considering the age of yours. Thank you!

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