Window will not stay up

A quick learner from Detroit says:
It's a double hung window and it won't stay up unless we prop it open with a stick. There is no marking or numbers on the existing hardware.
User submitted photos of a window balance.
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Paul from SWISCO responded:
Thanks for contacting us. Please see our 16-024 mounting bracket and 15-087 coil housing pivot. These appear to match the hardware I see in your pictures. I would also recommend our Series 180 coil spring. You will need the number printed on your old spring to determine the best Series 180 option.
A quick learner from Detroit says:
Thank you for your reply. The only marking on the coil is 729X. How do I determine what series to buy.
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Paul from SWISCO responded:
With a mark of 729X, I would recommend our S180-7. Each of these springs can carry 7 pounds, so if you have one on each side they would be able to support a sash weighing 14 pounds.
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