Heat oven to 350 degrees F.
Cook pasta according to package directions. Drain; rinse and cool.
Meanwhile, saute green pepper and onion in 1 tablespoon of the butter in skillet until softened, 3 to 5 minutes. Combine crabmeat, sauteed vegetables, black pepper, 1/2 teaspoon of the Old Bay seasoning in a bowl. Stir in egg, 1/2 cup of the milk and mayonnaise.
Melt 1/2 of the tablespoon butter in each of two 11 x 7 x 2-inch baking dishes. Spoon filling into shells. Arrange shells in each baking dish.
Melt remaining 2 tablespoons butter in saucepan over medium-low heat. Whisk in flour; gradually whisk in remaining 2 cups milk; cook until slightly thickened. Stir in Parmesan. Drizzle sauce evenly over shells. Sprinkle with remaining 1/2 teaspoon Old Bay seasoning.
Bake in 350 degree F oven for 30 minutes or until bubbly. Let cool 10 minutes. Makes 8 servings.