Tim - Your post and peek at your website made me reread my 1939 shop manual. Since it shows the gear shift on the column in the diagram, I didn't realize until now that this was the new option for the 39 passenger car only, and the standard was still on the floor.
Is your three speed snycronized? While I like having the low first gear, 7.23 to 1 is really a very limited gear option with today's roads and double clutching in a driver truck makes life very interesting at times.
It leaves me wondering though why all the trucks advertised come with the 4 speeds. Of course I guess I shouldn't wonder too much. There is a 40 on Ebay listed as a 1939 and after I told the seller the listing was incorrect they replied that "You must really enjoy these old trucks" without fixing the listing. Go figure.