In a small bowl, combine chopped eggs, mayonnaise, mustard, celery seed, and salt and pepper to taste. Mix well and set aside. Lightly butter bread. Spread half the bread slices with egg mixture, then top with watercress. Top with remaining bread slices, buttered side down. Press gently to seal. Wrap and refrigerate at least 1 hour. Cut sandwiches crosswise into 3 cm fingers.