Channel balance size and sash weight

A landlord from Boston says:
November 10, 2017
Hello this channel balance is to a old seasonall nuprime window. I would like to replace both ballasts
As you can see it measures 30
A landlord from Boston says:
November 13, 2017
Current ballast measures 30" . its stamped 30 10

I think your S351-30 series 351 channel balance may work. however i don't see the option to select 30 10 unless i select the 31" series 351 balance. (this would be to long
channel balance
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Paul from SWISCO responded:
November 17, 2017
Seasonal Nuprime window uses the Series 390 balance system with a 19-002 top and 15-001 bottom balance fitting. Please compare those attachments. Now you show a length of 30", so order the S390-30 side-load balance and choose 2910 for the spring weight. Someone might have been fixing the window and cut down a 31" balance channel to make it 30", which would explain why the stamp is 3010.
Top Sash Guide, SeasonAll
Top Sash Guide, SeasonAll
18-110
$1.70
30" Series 390 Channel Balance
30" Series 390 Channel Balance
S390-30
$11.30
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