This train set was entirely 6K afaik
In this case, a review found that besides the use of the wrong bolt, one contributing factor was the incorrect use of power tools in place of hand tightening, which led to damage of the screw threads and a false torque reading.
An initial Metro investigation completed within three days of the crash had concluded that the loose screw with damaged threads due to installation with a power tool was the root cause of the train coming apart.