Coins of Dancho and Issar

This is my chiddush; I believe it to be true. If true, it is yet another example how knowledge of archeology and ancient history can help us find the correct peshat in the gemara, and how lack of this knowledge can lead even Rishonim astray.

Here is the gemara, taking the text and translation from my Rif Yomi blog:

המראה דינר לשולחני ונמצא רע
תני חדא אומן ה"ז פטור הדיוט ה"ז חייב
ותניא אידך בין אומן בין הדיוט חייב
אמר רב פפא כי תניא ההיא אומן הרי זה פטור כגון דנכו ואיסור דלא צריכי למיגמר כלל:

If one showed a dinar to a moneychanger {who recommended it as good} and it was found to be bad:
One Tannaitic source taught: An expert, he is exempt; a normal person, he is liable.
And another brayta: Whether an expert or a normal person, he is liable.

Rav Pappa said: When that brayta taught that an expert is exempt, it is such Dancho and Issar {Rashi: renowned experts in moneychanging} who need no instruction at all.

I admit that I was stuck when trying to translate כגון דנכו ואיסור. And so I turned to Rashi, who said that they were experts in moneychanging. And I checked Soncino, who followed Rashi's lead and said the same thing.

But that Rashi says this does not impress me. No, I do not think he had a tradition as to the names of famous Jewish Babylonian moneychangers in the time of Rav Pappa. I think he looked for the interpretation of the difficult phrase which would make it read intelligibly, and decided that these must be the names of people.

However, I know that an Issar is the name of a famous coin. Now, this moneychanger might have been eponymous, but still, it struck me that perhaps כגון דנכו ואיסור might be the names of coins rather than people. And if so, Rav Pappa's statement would mean something different, but it would be more likely that his statement would make sense to people even far off. Then, I realized that the daled in דנכו could be separate from the root, and it is "de-Nachu," meaning "of Nachu." I also knew that on occasion in the laguage of Chazal, the khaf is used to render x of other languages. And so I formed my hypothesis: that the coinage of Naxo was some standard currency.

To test my hypothesis, I decided to search google for the words:
naxo ancient coin
and indeed, I found it to be true. Here is one such site:

Naxos coin - silver stater
The output of the Archaic mint came to an end in 490 BC, when the Athenians settled five hundred cleruchs on the island. The minting of coins, on the Rhodian standard, was resumed in the 4th c. BC. The surface of the silver and bronze coins was engraved with a depiction of the head of Dionysos and the kantharos.

Another example:

Naxos (wikipedia)
The coins of Naxos, which are of fine workmanship, may almost all be referred to the period from 460 BC to 403 BC, which was probably the most flourishing in the history of the city.

A search for Naxos coins yields 61,000 hits, and here is an article on JSTOR all about Naxos coins. Coins of Naxos are thus well known. And an Issar coin is also well known.

Rif presumably thought like Rashi, and so did not see the need to cite the end of the statement. But that statement is, in full, is:

It was stated: If a denar was shown to a money changer [and he recommended it as good] but it was subsequently found to be bad, in one Baraitha it was taught that if he was an expert he would be exempt but if an amateur he would be liable, whereas in another Baraitha it was taught that whether he was an expert or an amateur he would be liable. R. Papa stated: The ruling that in the case of an expert he would be exempt refers to such, e.g., as Dankcho and Issur who needed no [further] instruction whatever, but who made a mistake regarding a new stamp at the time when the coin had just [for the first time] come from the mint.

The contrast, which I did not see at first when I made my hypothesis, because I was learning straight from Rif, is between כגון דנכו ואיסור on the one hand, and coins just coming from the mint, on the other. It is not the contrasting experts in question, but a difference in coinage. The coinage of Naxos, and an Issar, is a well-known coin, so an expert should know it well, and would indeed be an expert. But in a newly-minted coin, even an expert might not have full knowledge of it and would be as an amateur.

Now, the setama degemara compares דנכו ואיסור to Rabbi Chiyya in the continuation, which might then have influenced Rashi. Perhaps the setama was incorrect in interpreting Rav Pappa; or perhaps the comparison is of Rabbi Chiyya the person with the established coinage of Naxos and an Issar (weaker). Tzarich Iyun, but I favor the former and so am not bothered by this problem at all.

This is then shocking. And this correct interpretation would seem to be against halacha as it has been established. (Of course, in monetary matters, what people expect is determinative in court-cases, so perhaps this misinterpretation should still be encoded as halacha.) But it is nonetheless shocking. Well, not for me, because I am sitting on top of much more shocking (and straightforward) interpretations of gemara. But what do you think about this one? Correct? Incorrect? If correct, what does this mean in terms of the accepted interpretation and in terms of halacha?