A Koban (小判) was a Japanese oval gold coin in Edo period of Japan. It is equal to one ryou (両), another early Japanese monetary unit.
Kobans were used as points to collect in order to reach a high score in the minigame Gold Rush, where Seija Kijin had the Miracle Mallet (Real) item that's able to destroy danmaku and convert them into kobans at the Hakurei Shrine.
The game converts the player's score into modern day yen, by using the sum "(Total # of koban) × 8932".
Kobans are also used by Shinmyoumaru Sukuna in Urban Legend in Limbo to attack her opponents. When the player presses the projectiles, Shinmyoumaru uses her Miracle Mallet to spawn some Kobans.
- Wikipedia's entry on Koban