Lianna writes:

Japan started minting its own coins around 700 CE, heavily based on Chinese coinage. These first coins were primarily round and made of copper, with a square hole in the center and four characters on the obverse. To the untrained eye, they are almost identical to Chinese cash coins.

Japan introduced a trimetallic system in 1569, but precious metals weren't coined originally. Instead, gold and silver dust were measured, then sealed into bags that were traded. This was clearly inconvenient, and both metals were ultimately coined into a variety of shapes.

Commerce became based on the Ryo, a base unit known, when coined in gold, as a Koban. Both silver and gold were coined into Bu and Shu denominations; one Bu was 1/4 Ryo, and one Shu was 1/16 Ryo.

However, the precious metals market in Japan was different than in the rest of the world. They completely closed themselves off from the world from roughly 1639 until 1853, when Commodore Matthew Perry from the United States landed on Japanese shores.

By the 1800s, most Bu and Shu coins were rectangular with sharp corners. These are Japanese bar money.

To visit the Rectangle Coins website, see:
The Rectangle Coins: Japanese Bar Money(https://rectanglecoins.com/)