Need replacement spring - no number

A quick learner from 29681 says:
I am trying to figure out which spring to buy but there is not a balance number on the spring. This is a single hung vinyl window installed around 1998.
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Tom from SWISCO responded:

Your pivot shoe is an older version of our 15-180. Although yours isn't made anymore, the more modern style we carry should work fine. We also sell the Series 190 coil spring to go with it.

Since there's no stamp on the old spring, I will need you to tell me the exact weight of the sash. I'd also like you to confirm that you only have one spring on each side of the window. That's the only other way to determine what Series 190 option will work.

A quick learner from 29681 says:
The sash weights 17.8 pounds. There is one spring on each side of the window.
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Tom from SWISCO responded:

Thanks! In that case I recommend the S190-940. These carry 9 pounds each for a total support of 18 pounds, which should be fine for your sash.

A quick learner from 29681 says:
Thanks Tom. Do you recommend me purchasing both the pivot shoe and coil springs for both sides?
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Tom from SWISCO responded:

Yes, I think that would be a good idea in this case. 

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