The fist to be how it were mentioned until now and the second to be how neither it were mentioned in a past but not now, this second could to be used if you will say something that show up contrast to the initial idea, exemple: "As mentioned in Section I, Chiba is a prefecture of Japan, but...(show up other idea)" Would be more convenient if you've used "on" instead "of", because it could seggest that Chiba would be prefecture of all Japan. ^^ Good lucky with English. Im fan of your country culture.