Double coil balance

A quick learner from Glens Falls NY says:
It has a #7 stamped on the spring. Appears to be your 171 series but l'm not sure. Right side as you're looking at the window.
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Bob from SWISCO responded:

This looks more like our 15-175 Pivot Shoe, which uses the Series 152 Coil Balances. A stamp of 7 on the coil itself would indicate you need an S152-7 7 lb spring. Let me know if you have further questions. 

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