... a dual denom on some coinage, that sounds so farfetched
And it didn't happen either!
This pattern was the result of the US participation in the International Monetary Conference held in Paris in 1867. That was two years after the formation of the Latin Monetary Union (LMU), with member states France, Belgium, Italy, and Switzerland. The conference was a French initiative with the purpose of discussing the possibilities to extend the LMU to a monetary unification "among all the civilized states." While in LMU the base denominations were 5 francs/lire in silver and 20 francs/lire in gold, the French delegates at the conference proposed a base denomination of 25 francs (in gold). That would be close to 1 British pound and 5 US dollar coins. The British and Americans would only have to adjust weight and fineness of their coins a little bit ...
. Not surprisingly, the British were not the least interested in changing their traditional pound to favor any ideas from the French, and the US was not overly enthusiastic. That not even the French authorities were united in their view of a more global monetary union didn't help either (the unification ideas were supported by the Ministry of Foreign Affairs and the Council of State, who favored free trade, but were opposed by the Ministry of Finance and the Bank of France, who were concerned about the effects of tying the French economy to that of less stable countries).
Anyhow, the US participation resulted in this pattern coin, but with discussions not really leading further and especially the UK opposing the whole idea, it didn't fly. And with the defeat in the Franco-Prussian war 1870, causing the end of Napoleon III and the 2nd French Empire, France became concerned with other things than trying to unite the world under a French-led monetary union.
Interestingly, there was also another pattern coin a decade later, the "Stella" 4 dollar coin (Wikipedia article
), which would correspond to the traditional 20 francs coin. It would have been used if the US joined the LMU (which was still in effect), which seems like an even more odd idea ... But Liberty undoubtedly looks better on that one!