I have always believed that Obama Senior’s visa is essentially a special diplomatic visa issued to a future member of the Kenyan government sent here specifically for an education.
I have been told that being sent here to learn the skills necessary to be a part of their future government does not rise to the level of a diplomatic visa, who knew?
It is likely that Obie Sr enterd the USA with an ordinary passport and either a Student F-1 visa or Exchange Visitor J-1 visa depending on where his funding came from such as himself/family and friends or a government organization etc.