Just as Federal POW Records during the war or vice-versa the opposing side had to rely upon what the prisoner told them about his place of birth or age or residence. Other things like height or color of hair was obvious.
According to the California, Prison and Correctional Records, 1850-1951: the best physical description of Black Bart yet.
C. E. Bolton alias Black Bart said he was born in New York and began his incarceration for the crime of robbery on November 21, 1883 for a period of six years. He was noted as a laborer age 48 and was five foot seven inches tall with blue eyes and gray hair. He was discharged on January 21, 1888. It was also noted he had square features and two upper front teeth gone on right side and two lower centers. It looks like the word Shield on right upper arm. Gunshot wound opp? (opposite?) naval on right side. Heavy tuft of hair on breast and scar under left wrist.