Window spring mechanism for a single hung window

A quick learner from Seattle, WA says:
I need a replacement part for a 5-6 year old single hung window that has hidden spring hardware at each side. One fell out. Number on it is 39DS HM 38 It is a spring inside of a galvanized channel. The channel is about 102 cm long.The spring is 58 cm long when not extended. I would replace the units on the right and left side of the window. Please let me know if any of this makes sense.
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Tom from SWISCO responded:
We should be able to help, but we will need more information. If you can, please remove the balance from the window and show us a couple pictures of it. Specifically, we need to see a picture of each end.
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