Might be an experimental one, but I've never seen any of the 311 series with an original vent rib. They saved that for the "Fox" Model B that is a gussied up 311.

Suspect it was added by somebody that liked your 311 as much as you do!

Any markings on it? Try a magnet, is it aluminum or steel? "Glued" on, screwed on, or soldered? All these details would give you clues (as would a tiny stamped name like "Simmons" or "Poly Choke".....common aftermarket brands).