On 22 September 1921, the day Walker and a number of other slaves were due to be processed by the Mexico City Manumission Bureau, a mob led by the Sons of the Wilderness Walk appeared in the plaza outside the building, threatening to destroy it.
Before they could act, Calles himself arrived on the scene and entered the building. He met Walker, who was due to be processed first, and left the building arm-in-arm with him. The two men made their way to the center of the howling mob, and stood in silence while the jeers reached a crescendo, then subsided. Within half an hour, the members of the mob had drifted away, leaving Walker, Calles, and some twenty reporters alone in the plaza.
Sobel's source for John Walker's manumission is Miguel San Martín's The Bloody Season (Mexico City, 1930).