What is "O" as a CountyFIP? + Why is largest county missing?

Question: The most populous county in Georgia is Fulton County (CountyFIP 121). However, using “Analyze Data Online” in IPUMS USA, and entering CountyFIP as the column variable, BPLD as the row variable, and the StateFIP for Georgia (13) as the Filter, I received 2 curious results: a column with “0,” and none for Fulton County. And there are far fewer counties in my columns than Georgia’s 159 counties - just a few dozen. What am I doing wrong? Does “0” just equal “everything else”? Why isn’t Fulton County there?

About half of US counties cannot be identified in the ACS public use microdata sample data. IPUMS identifies counties, where possible, from other low-level geographic identifiers; the lowest geographic identifier available in the public use data for 1950-forward is a public use microdata area (PUMA). I am linking a blogpost on the topic; the COUNTYFIP description page includes a spreadsheet of identified counties in the 1950-forward data as well. Fulton county is not identified in the 2012-forward ACS data; it can be surprising when a populous county is not identified, but occasionally the way PUMA boundaries align does not permit for identification of even very populous places.

Thank you very much.