<prompt> Translate the following text to EnglishSafu ya sita, iliyo na lebo Unknown inaonyesha baadhi ya barua ya darasa la kwanza ambayo haiwezi kutambuliwa na yoyote ya sekta nne zilizotajwa hapo juu.  <response>: The sixth row, labeled Unknown shows some First-Class Mail that cannot be identified with any of the aforementioned four sectors. 