AP

The 1950 census asked foreign-born people whether they were naturalized, with Yes or No. A third answer, AP, marked someone born abroad or at sea of American parents, who in most cases was a citizen from birth and never needed papers.

The 1900 census handled the same case in the birthplace column: the enumerator wrote the country and then Am. cit., for American citizen. Either entry tells you not to waste time hunting for naturalization records.