coil balance

A quick learner from watertown,ny says:
# 3 on back
pat.# 6,983,513
# 5,371971
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Bob from SWISCO responded:

This looks like an older version of a Series 140 Coil Balance system. Now, if the '3' is on the coil spring itself, then you'd need an S140-3 3 lb system. If not, and you're uncertain what spring weight you need, obtain the exact weight of the window sash. You can then divide that weight in half, and that will tell you the spring weight you need for both sides of the sash. 

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