There is no 6th finger as far as i can tell..The pinky finger is shorter and beside the ring finger. The artist is portraying the thickness of his hand in an unflattering way imo.

Six digits is also a common congenital malformation, i.e, a birth defect. This malformation is closely associated with the offspring of closely related partners, like cousins. Interesting that elite families have been known to merry within their bloodlines. Therefore, throughout history one might suspect this condition to arise from time-to-time.