Among the American casts, how many were uncredited on episode ID tt0629228?
American refers to birth_country = 'USA'; cast refers to category = 'Cast'; uncredited refers to credited = ''
SELECT COUNT(T1.person_id) FROM Credit AS T1 INNER JOIN Person AS T2 ON T2.person_id = T1.person_id WHERE T1.episode_id = 'tt0629228' AND T1.category = 'Cast' AND T1.credited = 'false' AND T2.birth_country = 'USA'