Non tilt channel balance

A quick learner from Sacramento, Ca says:
Hello, just need some guidance as to which replacement part I would need to order. The channel (minus the plastic feet) measures 22" long, 1/2" wide, and is stamped with 21 D (i think). From what I can tell based off the pictures on your site, I would need a 22" series 350 , is that correct? And if so, what "option" do I choose? Wasn't sure what those meant.

Thank you for any help.
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Tom from SWISCO responded:

You nailed it. S350-22 is your balance. They used to label their options A, B, C... but now they label them 10, 20, 30...

D would correspond to 40, so choose 2140 from our options.

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