Just found out that the rag refers to a headscarf in a custom (possibly among escaped/freed slaves) at a time when young men were sometimes kidnapped and taken to plantations by slave masters, where an older female relative or grandmother in a house would take a piece of cloth and tie it on her head as a scarf during the night to keep away the duppies. She would pray over the scarf, then give it to her son or son-in-law to keep in his pocket when he went to the farm in the hope that he wouldn't be captured. The boy in the song is talking to his mother about his father who was captured, and Mattie, or Miss Mattie, is his grandmother.