Sen (Currency)

Sen, the equivalent of 1/10 cp. Comes in coins starting with small brass 1 sen coins (uncommon), small copper 10 sen coins (common), medium silver 100 sen coins (common), medium gold 1000 sen coins (common), and large ringed silver/gold 10,000 sen coins (rare). These coins all have a small hole (all the same size) in the centre of them. For larger transactions signed notes are often used for the initial transaction, followed by payment of bags of 1000 sen coins days later.
The majority of the materials for sen are mined and precessed in the Tokugawa mountains, at which point they are shipped en masse to either Edo or Nagaoka (by land or boat respectively) where they are minted. 1 sen coins cost more to make than 1 sen each (about 1.290 sen) and thus are rarely produced, however, for reasons of tradition their production has not stopped completely.
For context on how large these coins are, a 1 sen coin has a diameter slightly smaller than that of a dime, and a 10,000 sen coin has a diameter slightly larger than that of a quarter.