There were actually several examples of the country being called “Republic of Kenya” in 1963. These were what...lucky guesses?
>>”they became a republic on Dec 12th 1964, exactly a year after independence. They even released 5 postage stamps to announce and celebrate their becoming a republic.”
Do you think that they maybe printed those 5 stamps BEFORE December 12th, 1964? Maybe the certificates of registration early, also. And began using them early, too.
The path from Colony to independent Nation had been, for them, long, bloody, (MauMau) and (toward the end) structured.
During the final year, waiting for states, they could begin using the new form, either officially, or as a gesture of defiance.
DG