Herring Sandwiches

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4 thin pickled herring fillets (about 3/4-pound), cut into 1/2-inch pieces

1 small potato, peeled, boiled, cut into 1/4-inch cubes

1/4 cup finely chopped onion

2 tablespoons finely chopped dill pickle

1 small apple, peeled, cored, cut into 1/4-inch cubes

2 tablespoons mayonnaise

Salt and freshly ground black pepper

1/2 stick butter, room temperature

6 thin slices black bread, halved

Green leaf lettuce