In a large bowl, cream together the butter and sugar until light and fluffy. Add the eggs one at a time, and beat well after each addition. Add the orange rind and vanilla.
In a separate bowl, combine the flour, salt, cinnamon, cream of tartar and baking soda. Add the dry ingredients and sour cream alternately to the creamed mixture, beginning and ending with the dry ingredients.
Stir in the nuts and plums. Divide the batter between the two loaf pans.
Bake for 50 to 60 minutes, or until a toothpick inserted in the center comes out clean.
Let the pans cool on a rack for 10 minutes, then turn the loaves out of the pans and cool completely on the rack. Makes 2 loaves.