"C4" suspension code on my fender tag

I have a 1968 Dodge Dart GTS 340". 9/13/67 build date. I'm in the process of restoring it. On the fender tag under the upper case "C" there is a number "4". Heavy Duty suspension is all I turned up. Is that on all Dart GTS's or is it something specific? Didn't find a whole of detail in the David Wise book I have.
Did these cars ever come with a 1-1/8" front sway bar from the factory? When I bought the car there was one in the trunk along with other scattered parts.

Thanks
Mike