I had thought there were critical differences in the shape or size of the blade when it came to the difference between the Green Dragon (sometimes given as Black Dragon) kwan dao and the Spring Autumn Knife. The Spring and Autumn period in China was likely from 771 BC to 476 BC. There is some dispute about the date boundaries, so one could safely say 800 BC to 400 BC. If the esteemed military theorist Sun Tzu really lived, it is likely it would have been in this period. The era takes it name from The Spring and Autumn Annals, which chronicled the history of the state of Lu between 722 and 479 BC. Tradition awards Confucius with authorship.
Noted martial arts experts David Chang and Wing Lam are of the opinion the only real difference between the two weapons if the dragon’s mouth used where the blade meets the handle. It is not clear how historical this difference is.
Curiously, in other work, after a major outbreak of some viral hemorrhagic fever like Ebola devastates China (and the rest of the world) mathematical projections indicate that in many scenarios after losing a billion people China would tend to resemble what it was like 2500 years ago. Other countries do much worse. A very high price to pay for not pursuing vaccines or cures or even extended research on the causes underlying viral hemorrhagic fevers.