Thanks David! News Gothic could be it. The font/type face appears to have been available in the time period. I had thought about but rejected the Leroy lettering sets based on the 3 but you could be right. At the time, other companies manufactured the key and other devices that use the same lettering. I remember some shortwave receivers and a direction finder receiver that I was given when I was about Ian's age. They had seen action in WWII. They had the same or nearly the same lettering on them. (I wish I still had them but they got passed on being too heavy to move.)
I guess I'll leave the labeling as it is on the key now, though. Your clues led me to find a set of fonts created from the Leroy lettering guides. My next overly detailed model might be the scriber and of course I'd need to model the templates. π