31 inches long; spring and pulley system; 2930 backward "C"

A handy person from Florida says:
I need to replace (8) of the 31"x 1/2" springs pictured, with the 2930C marking.

I also need (8) 31"x 1/2" with the marking 2910T for smaller windows.
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Bob from SWISCO responded:

This appears to be either our S351-30 or S390-30. The only difference between these two channel balances is the depth of the bottom nylon shoe, or 'heel-to-toe' measurement, so compare both with your old hardware to see what matches best. Choose stamp options of 2930 and 2910, respectively. 

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