need help

A quick learner says:
My windows are circa 1960. Below are photos of the size and type currently installed. Since I'm between sizes offered by Swisco, I'm wondering if I should order just larger or shorter than current? Current measures about 30.50 - 30.75".
Thumbnail of the uploaded file named spiral balance tube
Thumbnail of the uploaded file named spiral balance tube
Thumbnail of the uploaded file named spiral balance bracket
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Paul from SWISCO responded:

Thanks for contacting us. For your wood sash, you can use S500-31 balance with 17-012 bracket that is sold separately.

Below is a generic direction for the Series 500.

First, drop the balance behind the sash and screw it at the top with the 33-017 screw. Raise the sash and have someone hold it up, while you place the 17-012 bracket under the sash and have it corral the spiral. Screw the bracket to the sash. Continue holding up the sash while you bring the spiral up to the bracket. Now turn the spiral clockwise with the 75-005 tool about four turns. Then lock the bow tie at the end of the spiral to the 17-012 bracket. Do the same on the other side and try the sash. If it creeps down, give the spiral another turn.

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