I suspect the koku is a bit different, in that it's a persistent currency with a defined value of 1 certain quantity of rice, rather than a transient currency.
I suspect as a practical issue, being able to
actually redeem the currency for rice was erratic. If you have currency nominally backed by something else, you have two choices:
- Actually store enough of the material to pay back the currency.
- Just keep enough on hand to handle the expected rate of withdrawal. This may result in the bank defaulting.
Option 1 sort of works with precious metals (though it's not great even there), option 2 is mostly what actually happens. The problem for grain is that the money supply shrinks in exactly the situations where you want the money supply to expand.