Bob's 4145 number matches what I have found but there is more to the story. The following is from the July-August Issue of "Ford Life" magazine, Volume 3 Number 4, titled "The 1928-34 Open Cabs". This is the only good article I have seen on the open cabs since I have owned mine (1971). That number is world wide production, domestic production was 2637 bodies. Also The author the states that only about 40% of the bodies ended up on the pickups, the rest going to the big trucks. This gets us to 1055 1931 Roadster Pickups.
Charlie Stephens