Window Balance Replacement

A contractor from Maine, USA says:
The window balances have a plastic rod through the center of a spring that hooks onto an end cap that screws into the window frame. The spring attaches to this plastic end cap by an ear sticking out of the side. There is a plastic slider on the bottom of the spring that attaches also with an ear, and it has a string attached to it that goes to the socket that the window base fits into.

The string has a plastic oval on the end and goes under a couple of fingers on the socket. The channel is covered with a vinyl snap-in cover to keep the plastic rod from sticking out and hindering the window. The windows are double-hung and tilt in to clean. They are a little over twenty years old and I don't know the brand name. Of the twenty-odd windows in the house, all of the most frequently used ones have failed, but the only thing wrong is the balances.
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Casey from SWISCO responded:

Hello,

It looks like have an older rod balance that is no longer made but our S770 balance would be a 1:1 replacement as long as the terminal clip at the end of your pulley matches the 15-043. If this is the case, you'll need to measure the length of an undamaged balance and confirm the weight of your window sash to choose the corresponding length and strength options. Thank you. 

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