Replacment andersen 9#27 sash balance

A quick learner from illinois says:
I need to replace the sash balance in my Anderson windows. As best as I can make out, the number on the sash balance is B29 9#27 985. Which replacement model can I use, and should I replace both sides. Only one side is broken. If I do need to replace both sides. Please tell me which model number.
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Tom from SWISCO responded:
Thank you for posting! Please see our 24-939 Andersen Sash Balance. This can replace balance boxes with the stamp 9#27. It may be a good idea to replace both sides, yes. The 24-939 can work for either side.
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