Agreed on diamonds and gemstones. We're not looking at a very high-volume in circulation, though, so although they would be widely-accepted, they wouldn't be widely/frequently used on a day-to-day basis.
Ammunition might work but not everyone would want ammunition. It's essentially a barter item, which relies on the double coincidence of wants, where both parties want/need the item that the other is offering for trade. In the T2KU, ammo would have value, but not to everybody. Although folks with weapons or who deal regularly with such folks might value ammo as currency, others who don't likely would not. Currency is a universally accepted medium of exchange.
The Franc is an interesting idea. I think it would see fairly widespread use in regions bordering France (or French colonies) but I don't see it being used much beyond that. Poland, for example, is too far away from the issuer/guarantor/regulator. Maybe after a few years, if France does indeed become the major power in Europe but not really until.
I don't know much about Wampum, but shell beads were used as a kind of fiat currency by Native Americans and Dutch traders in the New Holland area in early colonial times. There was even an exchange rate between Wampum and Dutch coinage. What could be the Wampum of the Twilight world?
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