Abstract: A workpiece holding assembly for a machine apparatus is provided having an adjustable rail clamp attached to a rail which is secured to a work table of the apparatus. The rail is provided with front and back side referencing surfaces and the rail clamp includes an inner channel having an inside wall which comprises a reference surface for engagement with either the front or back side surfaces of the rail. The rail clamp includes a rail fastener means which functions to create a downward and inward force component which securely draws the inside wall against the rail reference surfaces to ensure a solid rigid connection. The rail clamp further is provided with a leveler connector means providing a quick release with a leveler head. The leveler head is provided with a connector portion that forms a corresponding connection with the leveler connector means.