#170 coil balance

A contractor from Bellflower, CA says:
Hello,

I have a customer with a Certainteed double hung window with what appears to be a #170 coil balance. However, the only stamp on the balance that I can locate is 10 J. Can you make any sense of this?

Thanks for your help,
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Bob from SWISCO responded:

I am afraid a 10 pound version of the Certainteed springs is not available. There's a solution however. If this is a Certainteed Coil Balance system, and the single spring is stamped with a 10, then what we recommend is using a S170-405 5 pound spring and S171-405 5 pound spring in tandem on each side of the window. This will equal 10 pounds of support per side, for 20 pounds of support overall. The video below explains how these parts work together. 

SWISCO S170 and S171 Coil Balance Assembly
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