These stone daggers are deemed imitations or models of Liaoning and Chinese bronze ones that are known to be distributed from the Liaotung region to the Korean peninsula. In Japan, they are believed to have been transmitted from across the seas, at a time when bronze daggers were extremely rare, primarily to areas near the Korean peninsula such as Tsushima and the northern part of Kyushu. This corresponds with the period when paddy agriculture first began in northern Kyushu, and once metal items started to diffuse in numbers from the end of the Early Yayoi they gradually disappear.
The daggers were found in three of the four wooden coffin burials, some were lined up along with stone arrowheads beside the interred, while others were placed on the torso or atop the coffin lid. Each jar-shaped vessel was located on the eastern side of the burial, it is thought that the heads were likely also orientated towards the east. The find throws light on Yayoi period weaponry as such stone daggers are known to have been used in warfare, as stone dagger tips have been found embedded in Yayoi skeletons.
On the significance of the stone daggers:
“In considering how the group which brought the stone daggers to Zasshonokuma was related to the dawn of Yayoi culture, we are provided with a splendid opportunity for reexamining the problem of the start of the Yayoi period, from the vantage point of East Asia” — Horizono Takashi
Other items found in wooden coffins at the Zasshonokuma site included and stemmed stone arrowheads (see photo above) and a Yayoi jar (see photo below)
Source: Zasshonokuma (Archaeology.jp website)