See no. 5160 on the long thread: http://www.freerepublic.com/focus/news/2306351/posts?q=1&;page=5151
A newspaper article from the Times of London, Oct. 9, 1963, using the term Republic of Kenya. This was the moment that Kenyatta’s party announced their intent to take the country independent, to reject the transitional form of government. It lead to the Dec. 12, 1963, constitution.
They considered themselves a republic from the point at which they declared their full independence from England (while remaining in the Commonwealth). England did not fight this declaration of independence. It was something of a slap at England since “republic” had always had anti-monarchical overtones, hence most other independent countries in the Commonwealth opted for Dominion or Commonwealth. But Kenyatta and his backers clearly chose “Republic” at the moment they announced their independence in Oct. 1963.
No, the Dec. 1963 Constitution did not use the term. But the term was being used by the leadership of independent Kenya from October 1963 onward. It is not unreasonable to assume that forms were printed with “Republic of Kenya” on them during 1964.
Whether this document is authentic or not, we still do not know, but the use of “Republic” does not prove it false.
I agree, it's possible a bureaucracy enthusiastic for the Republic changed the letterhead early. The only way to tell is if someone produces some other BCs issued around the same time.