The 4-speed according to the 51 Salesman data book is option number 318B HD synchromesh. In 1951 this otion priced out at $41.55 delivered. I am not sure rare is appropriate in Utah where I live the 3-speed seems to be more rare and the 4-speed common 60-70% of trucks I see here carry this option so it may be more of a regional ordering preference. Looks like a very nice truck best of luck and enjoyment with it. One more comment the trucks don't have a lot of information available that you can track in the way of options and equipment so take lots of pictures and ask questions on items you may be wondering about someone here will be able to help with the answer.
