My guess would be that in the '30's and '40's, people were buying trucks to use them as.. well.. trucks.
A truck back then wasn't a status symbol, and people didn't buy a truck to 'look cool' or because they were trendy.. they bought a truck because they needed a truck to do truck type stuff.
So while the three speed might have been the 'standard' configuration, more often than not, your typical truck buyer of the day was going for that 4 speed 'option'.
In fact, it's pure marketing genius that can still be found in new vehicle sales today.
You advertise the vehicle as 'standard' or 'base model' at a low price to get the customers in the door, then tell them.. oh yea.. by the way.. if you want silly things like HEAT you're going to have to pony up for the 'heat' option.
Ohh.. planning on using it as a real truck? Well for a couple bucks more you can get the 4 speed option!
That kind of thing.
Same goes for the '47 thru '53 trucks.. the '5 window' was supposed to be an 'option' but I see way more of those on the road than I do three windows.
It was just a popular option.
