I've watched this thread with some interest...
With regard to the 747 and the E2, the end plates on the vertical stabilizer are to enhance directional stability, as mentioned above. The reason for the endplates, as opposed to the std config, is due to the fact that the space shuttle on the 747, and the radome of the E2, certainly impedes airflow to the vertical fin...Grumman chose to build 1 tail, and used it for the C2 and E2 for the same reason as described below:
The 1900d is, as most know, simply an evolution of the B1900C, and of the King Air series. In particular, the tail shared by the -200, -300, -350, -1900c and -1900d is the same tail as introduced by the F90 in 1979, for a 750 shp application. As the a/c was lengthened to the -200 and above, the same tail worked simply b/c increasing the length of the a/c compensates for tail area.
(Witness the 747SP, the "short" 747 that has one of the largest vertical tails in the history of a/c production, to include the C5, AN 124, and B52.)
As horsepower increased, the vertical fin area reached a point where it HAD to be increased for the 1900d. So why not use the same part and not have to retool? BINGO.
So for the 1900d, which has a great deal of single engine Reserve HP to accomodate and engine out situation), extra vertical surface was needed, hence the endplates on the tail. It's also greater than 12,500 lbs, which places some additional constraints on the design.
The directional stab topic surfaced about 2 wks ago; my assumption for the hozizontal surfaces on the 1900d is the same as the for the endplates...it's cheaper to build the same vert tail for a variety of a/c, and simplify the 1900d as needed to account for its specific requirements in the pitch and yaw (and therefore roll) axes.