You may be referring to the "Consular Report of Birth Abroad", Form FS-240.
I am unfamiliar with the laws of Northern Rhodesia/Zambia, but it appears that they granted you a kind of conditional citizenship that had to be confirmed by you before turning 18. Their laws are their laws, this notice by 18 is NOT US law. BTW, you are a US Citizen by birth. Most would say that makes you a natural born citizen, but there are those who insist that there is a kind of super citizenship that is required for those who aspire to the Presidency.
it was a Certification of Birth issued by the Department of State, Foreign Service of the United States of America with a raised seal.