Balance Question

A quick learner from USA says:
What is the difference between 407 Series 170 and 407 Series 171? How can I tell which one I have? I only see "407" stamped on the spring. Thank you.
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Paul from SWISCO responded:
The S170-407 is meant to replace balance systems that use only one coil spring on each side of the sash. The S171-407, on the other hand, has an extended "tongue" so you can piggy back the spring on top the S170-407 in the cases where there are two springs on each side of the sash.
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