Sash support spring for old wood windows

A home owner from USA says:
Would you please help me find window sash support balances for my windows? The house was built around 1953.

Attached are two photos showing the bottom of the lower sash and a view that shows the lower part of the top sash. The second picture shows the plastic cover (broken) that surrounds the spring. My measurements are 0.55 to 0.6 inches for the plastic and 0.41 inches for the spring. Fully extended the support balance is about 36 inches from the bottom of the lower sash to the top of the support.

Thank you very much.
User submitted photos of window hardware.
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Tom from SWISCO responded:
Thanks for posting! Looks like an old spiral balance. For wood windows like this, we typically recommend our Series 400 balance if your tube is 3/8" in diameter, or our Series 500 balance if your tube is 5/8" in diameter. What is the diameter of your silver tube? It would also be used to determine length, though I know its broken so it may be tricky. Just try to use your best judgement.

You would also need a new bracket to go on the bottom. See our 17-012 or 17-007.
A quick learner says:
Thanks Tom. I measured the outer tube and it is 0.55 to 0.6 inches. Your 500 balance is 5/8 inches which is 0.625 inches so I guess I need the 500 series. As far as length needs to be, I measured the length of each sash and they are both 20 inches from the top of the sash to the bottom. Is this sufficient info to determine the length of the balance? Thanks.
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Tom from SWISCO responded:
If you can't measure the length of the tube, a better method would be to only measure the height of the visible glass for the sash in question. Tell me what that is and we can recommend something for you!
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