Sanitation in Sampson County, 1914

Forgive me for bringing down the typically high level of discourse on this blog, but I have a question about toilets. The fascinating Map of Salemburg community: from April 15 to Oct. 1, 1914 looks at the changing sanitary conditions of households in 1914, most likely in response to an outbreak of hookworm in the area. The progress of each house is charted, as explained in this detail from the map legend:

Now I understand what the numbers mean, and the symbol for owner or tenant, but what I can’t figure out are the abbreviations for different types of privies. The letters included in the symbols are most often “F” or “E” for the privies examined in April, and mostly “B” for those examined in October. There is nothing else on the map to suggest that they might represent. Do you think these are just codes used by the sanitary commission, or do they stand for specific words? (Flush? Bucket?) Please, keep your answers clean.