We are making some circular arguments. "Fiduciary" = a promise taken as obligation. The fiduciary value of a 1575 Mexico 4 reales was that it, and all its brethren and those to come were in fact .931 fine silver. That got them into East Asia, but once passed from European to Chinese, into the melting pot they went, to be adulterated with alloy. They circulated as intrinsic metal. What was not accepted was any kind of subsidiary, token, or other fiduciary device from the King of Spain.
Where to begin?
Since clay tokens apparently symbolising quantities of grain, cattle etc are about 10,000 years old, and the oldest pair of scales I know of are less than 6,000 year sold – the evidence suggests that creditory instruments might well pre-date metallic payments. Since writing is only 5,000 years old, we are stuck at guessing on that matter.
What we can say with some certainty is the sort of fiat coinage system adopted world-wide in the 20th century (and the UK specifically in 1947), was basically the same as that launched in China by Wang Mang c. 14-23 AD. Fourth century Roman copper is surely best explained as fiat also?
Spain had a vast circulating fiat copper currency in the 17th century. Unlike early medieval Bokhara however, Spain was repeatedly devaluing it, so no sensible person outside Spain would hold it. Anyhow, since China had been issuing fiat money of its own, on and off, for more than two thousand years, its hard to see why they would want it from Spain anyhow?
They circulated as intrinsic metal
The crucial fact about the silver scycee issued under the single whip taxation system in China was the rate at which it was accepted in payment of taxes by local magistrates. Notional “melting charges” of between 10% and 50% were applied – meaning that the claim they circulated as "intrinsic metal" is false. At the crucial juncture in their circulation, payment of taxes, they passed at up to 50% below intrinsic.
That is surely why silver coin issue was so retarded in China - till around 1900? Why settle for maybe 6% every 50 years seigniorage on coins, when you can take 10% to 50% - every year!!! - on what you very misleadingly call “intrinsic metal” circulation.