Non tilt channel balance

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.