The ol' pre-1957 Jag MkVII, Cal Model, was the heater-delete version.
In a desperate attempt to improve reliability it was reasoned that removing the unnecessary heater core and lines (on essentially the India version of one of the first "World cars") couldn't hurt. Because it never rains in Southern California little attention was paid to weather (wind) proofing during the manufacture of the "Cal version" production run. So if you happen to find yourself in an ol' pre-1957 Jag MkVII, Cal version, hard-top or soft, somewhere that it does rain, you may get wet... and cold.
That big plastic knob was merely a concession to aesthetics (as were most switches and such) In a Huge conspiracy between the U. K. auto makers of the time, cars where sold as having features whose installations were never even completed beyond those very knobs, switches and dials!
As a result, the blame for the entire U. K. Motorcycle and Automotive Industries abysmal reliabilities of the era were blamed, Unjustly!, on a single company, Lucas Electrics.