A federal appellate court in New York has ruled that a Claude Monet painting worth $500,000 belongs to the New Yorker who bought it 30 years ago, not the original owner--from whose castle it disappeared during World War II. The decision by the 2nd U.S. Circuit Court of Appeals reversed a lower-court ruling returning the work, "Champs de Ble a Vetheuil," to a West German woman whose family had owned it since 1908. The three-judge appeals panel found that under New York law, Gerda Dorothea DeWeerth "failed to exercise reasonable diligence in locating the painting after its disappearance, and that her action for recovery is untimely." Noting that many prewar records concerning the painting have been lost, the court said late last week that "to require a good-faith purchaser who has owned a painting for 30 years to defend (against a prior claim) under these circumstances would be unjust."