Stir the sour cream, horseradish and salt together in a small bowl.
Using a butter knife or offset spatula, spread the horseradish cream on 1 side of each rye slice. Put 1 slice of roast beef on 12 of the pumpernickel slices. Finish each sandwich with a grind or two of pepper and the remaining bread. Arrange the sandwiches on a serving platter and serve immediately.
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