Not sure what size channel balance this is

A home owner from California says:
The channel is 1/2" by 21" . I am not sure which one of your models to go with.
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Tom from SWISCO responded:

Hello! Your balance is either our S390-21 or S351-21. The only difference between these two products is the style of the bottom nylon fitting. You'll have to measure yours to confirm which one to go with.

Regardless of which balance you choose, you'll also want the 2030 stamp option to match your old 20-3.

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