http://news.bbc.co.uk/onthisday/hi/dates/stories/may/27/newsid_2496000/2496069.stm
1963
I’ve been at it all day and this is all I can find.
http://scholar.google.com/scholar?hl=en&num=100&q=%22republic+of+kenya%22&as_ylo=1963&as_yhi=1965&btnG=Search
5th one down... there’s a reference to a book report with the phrase “Republic of Kenya” published in November of ‘64.