Still digesting ... and I see there's more to digest.
Say there's no fixed exchange between shells and silver, and it's determined by the market. Are they both money?
I think metals would be a "medium of exchange," and the shells would be money.
For taxes the government would want metal, labor, or goods.
Are both legal tender for settling all debts?
No, since shells are either impractical or perhaps not allowed with some usages.
Are there banks? Can they issue loans in both, or just one?
Few or no formal banks, but there are people and organizations interested in loaning money. Unregulated, so they'd do whatever they wanted. I'm hoping they'd be uninterested in working with shells.
Overall, I'm shooting for as little regulation as I can get away with without having the system bend itself to a different state.
A so like ancient China it produces all the goods it needs?
Needs, yes. There's foreign trade, but focused on luxuries and tightly controlled. (ATM all sea-going trade, for example, is conducted via gov-owned boats. Merchants bid for shares/cargo space.)
The land borders are porous, but there's not anything traded markedly different from interior products.
My guess is that shells wouldn't be at all popular outside the country ... though the tech is low enough that people, as they did historically, might give them significant intrinsic value as ornaments. But still ... not much. So barter would be common.
So I do see silver (or other metals) leaving the country one way or the other.
OTOH, what if silver and gold tended to come
into the country via foreign trade? Finished goods or commodities for gold and silver. Or "Take this vase, give us that gold, and we won't kill you."
But I suppose the gov. just seizing and running a lucrative mine is the most likely scenario, foreign or domestic. At least if it's reasonably close.
I'd prefer the gov. introduce precious metals into the system "organically". ie, somebody doesn't say, "I think we need to loosen the money supply." Rather, it "just happens." How that'd be, I don't know. Just paying for things? I suppose this is the "source" issue you mentioned?
I guess that would fit well with there not being much metal in circulation, since there isn't all that much the government would want to pay metal for. They'd employ a lot of labor, but want to give them shells.
What would they offer metal for? Wages for high officials, military officers, and tithes to temples.
Payments for skilled artisans for their labor.
Payments for slaves and other relatively expensive goods. (Quite a bit of the military equipment ... horses.)
Metal might be offered for foreign goods, then come back into the country when foreigners buy things.
And if the government doesn't set an exchange rate, then you will find that there will a lack of a merchant class because where is the motive? No matter how many shells you gather you will never be "wealthy" as I understand it.
My thought is the lower-order of merchants who aren't allowed metal could still become much more wealthy than the average person, even if the wealth is only in shells. Such merchants *could* boot-strap their way toward metal-using merchant via accumulating the shells to get something that can be traded for metal. Or bribing the right people.
That will subsequently prevent the country from creating more advanced forms of a monetary system.
That'd be super. Exactly what's wanted.
There would be merchants, but their aspirations would be to become something beyond merchantist. Their goal would be to facilitate more wealth for the existing gentry (shells for slaves, slaves for silver, etc) that the gentry need to show off to one another, in exchange for a spot amongst them.
Generally in an informal manner? (Not "I'm a local lord and these are the merchants I sponsor.", but rather, "I'm a local lord and these are the merchants who operate in my area, and whom I buy from. I'm glad I don't have to go fleece all the peasants one by one!")
It really comes down to how capitalist or not this world is. Where does status actually come from? Is it accumulated money or something else?
I'm thinking not very capitalist, in that the government owns a great deal, various religious organizations own a great deal, and aristos own most of the rest. Wealth tends to accumulate in the form of land or slaves or goods. There's little social mobility, and innovation is actively discouraged. (At least where it doesn't mean a great deal of additional wealth for someone already wealthy.)
Edit: "Crony Capitalism" might fit well. There's often a presumption of a need for government permission. "You're allowed to go here and trade silver for cattle ... you're not."
I don't think "state capitalism" fits. The central state is very powerful, but it has to share a lot of power and wealth with religious organizations and, to a lesser extent nobles (I think "nobles" are generally just families of great wealth.)