Seeking clarification of “F/O - not shown” for naturalization date

I am trying to confirm if my great-great grandfather actually was Naturalized or not. I have a form in the microfilm of Louisiana State Archives records with the name August Pierrote (born Auguste’ Constantine Pierrotte or Pierrotti) and for Date of Naturalization it has “F/O - not shown”. Does anyone (1) know what that means and (2) can guide me on finding any record that could confirm whether or not he was naturalized? 

He was born May 8,1830, but this microfilm form says March 16, 1830. He arrived in NY in June 1855 from Italy. He settled in Louisiana in 1862 in St. Landry Parish and he died April 30, 1892.