The radiator shutters were necessary if the tractor was dual fuel. If the WD was not dual fuel, generally the shutters were not purchased. Even so, most dual fuel WD's and WC's had the shutters removed and were discarded.
The horsepower on the belt of the WD was 34.6, the D14 was 34.1, the D15 Series I was 43.6 and the Series II was 46.2. The D14 and WD were pretty similar in capability. The D15 was a lot closer to the WD-45 (43.2 on the belt). They are all great tractors!