sash balance

A contractor from Ventura, Ca. says:
Channel is 9/16 wide and 34" long. End of channel to end of channel. It is stamped with 3390 33. It comes from a Jeldwin window Vertical sash only, no tilt. Please give me a price for 3 sash balance sets. I need to replace on 3 windows.
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Bob from SWISCO responded:

This looks like either an S380-34 or S385-34. The only difference between these two balances is the depth of the bottom nylon shoe, or the 'heel-to-toe' measurement, so compare both with your old hardware to see what matches best. Your stamp code of 3390 is from an old rating system we no longer use. To know what stamp option to pick, obtain the exact weight of the window sash. You can then match that exact weight with one of the weight ranges listed alongside each stamp option on the shop page. 

S380-34 : 34
34" Series 380 Channel Balance
S380-34
$11.28
VIEW >
S385-34 : 34
34" Series 385 Channel Balance
S385-34
$11.28
VIEW >
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How to Identify Replacement Channel Balances
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