Agreed here Denny. The more correct statement would be the standard transmission is the 3-speed but the optional 4-speed is a very common upgrade of that time. Here in the west I have never even seen a 3600 with the column shift three the 4 speed is/was the preferred way to order or spec the truck. I would also agree that about 1/2 the 3100's here are equipped with the 4-speed. I like Denny purchased my truck from the original owners family and it was factory original with the 4-speed. Here on this 1950 salesman order book you can see it was a $50.00 option, I think option is the key.
http://www.flickr.com/photos/advance-design_parts_co/8335509814/in/set-72157631872034068
