Britain was in the process of divesting itself of it’s colonies...it seems like it was doing it in an orderly way that the Foreign Office could handle. Kenya was pretty much at the bottom of the list due to the Kenyatta situation, Mau Mauism, etc. The new countries, although remaining part of the Commonwealth for economic reasons, wanted to emphasize their independence by calling themselves republics as opposed to dominions or establishing local monarchies.
If WND says they have examples like the one here...may put this ‘Republic’ issue to rest. Besides the Obots would have a heck of a time trying to cover up every 1964 document in existence if some do say ‘Republic of Kenya’ somewhere on it. LoL!