I don?t know whether this is genuine or not, and a specialist catalogue is really needed to see if it matches correctly to known examples. As I said, replicas are not uncommon.
However, I don?t agree with your assessment. I don?t believe it?s possible to say whether or not it is in fact silver from a photograph (although a replica could of course also be made from silver).
The website you linked to shows units of ?one lang? with weights around 38g but the weight of these pieces varied according to their denomination (and also a little as a result of changing silver prices and/or silver purity because they were ?bullion currency?).
Yang Hao?s translation tells us this example is a unit of ?5?, so it won?t necessarily be anywhere near 38g. It gets complicated because these bars (also sometimes called ?trays?) were linked to Chinese currencies, which in turn were linked to the unit of weight known as the ?tael? which was approximately 40g? but with actual currency values fluctuating for the reasons I gave above. My guess is that it was intended to be valued at ?5 tien?, the tien being the prevailing monetary unit in imperial Vietnam during the 19th Century, but that?s not the only possibility. One tien could be anywhere between 3 and 4 grams of silver. An actual weight of the piece would be helpful.
Also, the examples in your link are generally older pieces than this one. The workmanship varies enormously according to the merchant bank that produced them, and also progressively improved with time through the 1800s, which is why I said this would have to be a late example, so I was encouraged by Yang Hao?s translation that it has an imperial date span between 1885-1889.
It?s not ?way too perfect? for its age to rule out it being genuine. Even by the mid-1800s, workmanship standards could be pretty high, as evidenced by this 5 tien piece from between 1820-1841 (and later pieces can be better still):
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