I believe that sempaiscuba is probably correct with their answer. However, there is a slim possibility that the "M" stands for "manumitted", which is the technical term for a freed slave. It seems unlikely that the census taker would use an additional letter instead of just "F" or "S", but I suppose it is possible that they might have been trying to make a distinction between those who were born free and those who were born slaves and then later freed.

A simple transcription error is definitely the more likely scenario, however.

Out of curiosity, is the person in question listed as white, black, or mulatto? Because if they are listed as white, it is definitely a transcription error.

I believe that sempaiscuba is probably correct with their answer. However, there is a slim possibility that the "M" stands for "manumitted", which is the technical term for the act of a master freeing a slave ("the slave was manumitted" = "the slave was freed by his master"). It seems unlikely that the census taker would use an additional letter instead of just "F" or "S", but I suppose it is possible that they might have been trying to make a distinction between those who were born free and those who were born slaves and then later freed.

A simple transcription error is definitely the more likely scenario, however.

Out of curiosity, is the person in question listed as white, black, or mulatto? Because if they are listed as white, it is definitely a transcription error.