Need a whole balance assembly

A contractor from Salem, Michigan says:
November 16, 2018
The balance pictured is the one that came out of the window. The only markings I could find are "15," and "13," on the upper and lower coil covers respectively. I did order a couple of the s140 balances and they fit dimensionally, but two 8-pound balances are too weak for this window. I believe a 3 coil balance would fit, but I'm just trying to make sure I get the right part this time. Your help and time is greatly appreciated!

Thanks,

Brian.
A contractor from Salem, Michigan says:
November 16, 2018
Upon further inspection, I found a "6 K" stamped on the upper coil of each balance, but no markings on the bottom coil.
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Tom from SWISCO responded:
November 20, 2018

It looks like you have our 15-177 pivot shoe, which is compatible with our Series 152 spring. Since you have a "6" printed on your old spring, you'll want our S152-6. These can carry 6 pounds each, so assuming you have four holding up your sash then it's probably around 24 pounds.
Coil Balance Shoe, Slate Blue
Coil Balance Shoe, Slate Blue
15-177
$3.47
6 lb Series 152 Coil Balance
6 lb Series 152 Coil Balance
S152-6
$4.80
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