Abstract: A twistlock for securing a structure to a corner fitting having an ISO aperture. The twistlock includes a main body, a first clamping cam toggle pivotally mounted within the main body and a second clamping cam toggle pivotally mounted within the main body. The twistlock also includes a T-clamp screw having a body portion, a threaded end portion and a clamping portion, where the body portion extends through an opening in the main body and between the first and second cam toggles so that the clamping portion is positioned adjacent to an outside of a bottom wall of the main body. The T-clamp screw is rotatable to engage an underside of a wall in the corner fitting and the first and second cam toggles are pivotable to engage edges of the aperture in the fitting.