Series 351 balance channel spring

A home owner from Oviedo Fl says:

Could you assist me in determining which spring I need when ordering new series 351 balance channels for my windows? My windows are a single pane glass with the movable portion approx. 29x29 (or x 28). I determined that my balance channel length is 29" & ends indicate that series 351 is what I need, but my channels have no stamps or markings.

Thank you for your help. Ed
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Tom from SWISCO responded:
Thanks for posting, Ed. If your balances do not have any markings on them, then the only other way to determine what option you need is to weigh the window sash by removing it from the window and placing it on a scale. From there, you can cross-reference that weight to the chart of options we have under the S351-29.
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