The Photographic History of The Civil War: in ten volumes, Thousands of Scenes Photographed 1861-65, with Text by many Special Authorities, Volume 7: Prisons and Hospitals.
Francis Trevelyan Miller, Ed.

Provost-marshals.

It was not until 1863 that Negro troops were enlisted in the Union army. Properly led, they made excellent soldiers, but there were times, like that shown in this photograph, when they were difficult to handle. In idleness they always deteriorated in discipline. The accompanying photograph, taken after the fall of Vicksburg, shows one of the punishments inflicted on soldiers who had committed breaches of discipline. They were set astride of a plank six inches wide and forced to remain in this position, which was neither comfortable nor dignified, for two or three hours under guard. The Negro guard, clothed with a little temporary authority over his fellows, is apparently swelling with importance. The two Negro soldiers ‘riding the sawbuck’ look apathetic, but it is doubtful if they are enjoying themselves to any great extent.

Men who policed the federals—provost-marshals of the third army corps, December, 1863

‘Riding the Sawbuck’ at the Vicksburg guard-house