Certain teed Coil Spring

A quick learner from Downingtown Pa says:
I have certain teed single hung Windows. Each side take two coil springs 405J. What is the difference between the 171-405 and 170-405?
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Paul from SWISCO responded:
Hello! The Series 171 is specifically meant to be used in coil balance systems that use more than one spring. the Series 170 spring, which has a short tongue, would sit on the bottom, while the Series 171 would sit on top and use its longer tongue to reach all the way down to install.
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