Sanborn Fire Insurance Atlases, you know that the map keys changed a lot over the years.
We were recently fact-checking the location of a historic photograph and came across this former Fourth Ward public school building (304-310 N. Robertson) that had become the Hotel New World by 1909. "F.B" refers to the gendered space, Female Boarding or Bordello. When the school building was altered to serve the Red Light District, a cluster of cribs was erected along the Bienville Street side.
Image above: Sanborn Fire Insurance Map of New Orleans. New York: 1909. Vol. 2. Detail, as viewed via Digital Sanborn Maps.
For more information on the Hotel New World, see Alecia P. Long, "Sex and Tourism in New Orleans." Chap. in Richard D. Starnes, Southern Journeys. Tuscaloosa and London, 2003, pp. 26-27.