Tsukuriyama burial chamber, Okayama Prefecture, Japan. (last picture in above gallery, note again the “knobs”
Two megalithic monuments in Japan that defy logic
Ishi No Hoden monument
popular theory is that it was used as an astronomical observation point. Evidence for this perspective comes from the ridge line across the top of the rock which runs parallel to the mountain ridge in Asuka and lines up with the sunset on a certain day of the year called “spring doyou entry”, which occurs 13 days after the sectional solar term ‘Pure Brightness’. This day was important in the lunar calendar and for early Japanese agriculture as it signalled the beginning of the agricultural season. However, this perspective has been largely dismissed by scholars who do not recognise it as an ancient astronomical observing station.
In russian, but pictures are good. http://lah.ru/expedition/japan2012.htm