Interior: Knob Function on 2R5 dash

[over50]

On the instrument cluster of my 1950 2R5 pickup are a knob for the light switch (L), one for the Choke (C) and a third one of the same large size with two rectangles in white. (There is also another small knob for the dash lights.) I looked in my manuals including the owner's manual but can't figure out what the third large knob is for. It seems silly that I can't figure this out, but the knob shaft isn't connected to anything on the engine side of the dash. My only guess is that there should be a cable going somewhere to open ?? a vent but???

If you can tell me what it is for and where/how it should route, that would be great.
Thanks
Norm

[Dwain G.]

The two white rectangles are actually art-deco style lettering spelling 'O D' for Overdrive. It should operate a cable that runs down to the rear section of the transmission.

[over50]

Since the pickup now has a 259 and Flightomatic, I guess it is decoration. Since I have an electric choke, that makes two knobs that aren't functional.

Thanks
Norm