That is the access plate for the windshield wiper (above the windshield) from a truck. If Chevy, then probably '39/'46. Your photo is the back side. Two holes, one for on/off, one for speed control knob. The most common would be for the drivers side, much, much, much less common would be accessory passenger side. Other, non Chevy/GMC, trucks might have had a similar panel.
Mike I think you are right about it being a wiper motor access plate. And I never would have thought of that until you said it! :-) But it's definitely not 1941-46 truck. It's hard to find good pics on the net... but it may be from the style of cab used on the 1937 truck.