Your Montague Cub S28-6 model stove is an older one, and the Montague 15591-8 19 1/2" Rear Burner Assembly will not work with your unit.
Speaking with the manufacturer I've learned that they no longer make the venturi part of the burner assembly for your S28-6 stove. That said, Montague still makes the burner heads (part #3371-5), gaskets (#30924-9), and attaching screws (#6976-0). If your current venturis are still in working order you'll only need to replace the burner heads and gaskets.
Unfortunately, if your venturis are damaged you'll need an aftermarket part or to have one fabricated for you. Please give a member of our Tundra Restaurant Supply team a call at 1-888-388-6372 to discuss your options.