The literal translation of the title of this movie is: The Last Clue Of The Mystery Case.
The movie made in the early era of the reform is an example of the devastation the Cultural Revolution caused, which still lingered on after it had ended.
To start with, the time was wrong. The skull of Peking man was lost in December 1941. Before the Pearl Harbor, the Chinese government was already at war with Japan and felt that the skull was not save so USA agreed to help transfer it by loading it on the military train of US Marine Corps, because USA and Japan was not at war yet, and USMC military train enjoys the benefit of neutrality in China.
However, on its way, Pearl Harbor was attacked and Japanese forces in China acted simultaneously by attacking western forces in China, including USA, and the military train of USMC was captured by Japanese, and the skull was never found until this day.
During the Cultural Revolution, nationalists were demonized and all heroic resistance movements were lead by communists, who took no parts in finding the skull because they did even know anything about the skull and its importance. Therefore, the positive contribution of nationalists were ignored willfully, wiped from history lecturing in China. It was not until the era of reform did the Chinese government finally begin to restore history honestly as the way it really was.
However, by the time of this movie was made, it was still the early era of reform and history was not completely restored yet, so this movie included the fictitious plot of commie agents trying to locate the skull while in reality they did not even know about the skull, the nationalist agents trying to do the same but in reality did not either because they had more important job of fighting a war, and lastly, the collaborators trying to do the same but in reality that is not the case either because the skull was already in their hands.