Combine potatoes, cucumber, onion, celery seed, salt and pepper. Toss lightly together. Separate yolks from whites of eggs. Chop the whites and add them to the salad. Mash the yolks and combine with the sour cream, mustard, mayonnaise and vinegar. Beat well and toss lightly with the potatoes. Serve on crisp lettuce garnished with tomatoes and sprinkled, if you wish, with crumbled crisp bacon.