Because twelve out of thirteen Kinen mirrors, which were with the name of an era of China, were included in bronze mirrors produced in this period, these mirrors were used as references to estimate the era when the excavated tumulus or other burial goods were produced.
この時代の銅鏡の中には、中国大陸の年号が銘されている紀年鏡が13枚のうち12枚が含まれており、出土した古墳やその他の副葬品が製造された年代を推定するための参考資料とされている。