I was born to American missionary parents in Northen Rhodesia, now Zambia. I was issued a U.S. birth certificate, the year was 1960. Would be happy to scan it for someone who knows more about this than I. My parents told me that I automatically had dual citizenship because of place of birth, but would only have American citizenship at age 18 unless I declared otherwise. Dad is now gone and mom has dementia.
Interestingly, misplaced one copy of original - I have a second -when going through an International adoption process for our child ( was not returned by Homeland Security, although they admitted they had a copy of it !!! So suppose someone has it !!! ). Applied for a new BC, and was sent a new version signed by Condoleza Rice.
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.