Anything called Curaçao liqueur should be made of the zest or peel of Citrus aurantium currassuviencis, the Curaçao orange — an especially bitter variety of Citrus aurantium, the bitter orange. The Curaçao orange grows on the island of Curaçao in the Netherlands Antilles.
Surfin Curaçao liqueur is specifically distilled from of a maceration of the peels in an overproof spirit. It is considered the standard. Surfin is French for ‘superfine.’
Surfin Curaçao liqueur may be of any of several grades for sweetness, and intensity of bitter orange aroma.
In addition, surfin Curaçao liqueur of any of the above grades may also be either colored or left blanc (‘white’ or colorless).
The sweetest grade of Curaçao liqueur is doux (French for ‘sweet’). No Curaçao doux liqueur seems to be commercially produced anymore.
Less sweet than Curaçao doux is Curaçao sec. Sec is French for ‘dry.’ Early French Curaçao liqueurs were very sweet, and Curaçao sec may have been created as a later adaptation toward the Netherlands original.
Even drier and more aromatic than Curaçao sec is Curaçao triple-sec. Triple-sec is French for ‘triple-dry.’
The very driest and most aromatic grade of Curaçao liqueur is extra-sec (French for ‘extra-dry’). Though originally of a sweeter grade, Cointreau’s famous product has become drier over the decades (perhaps in response to the success of Cusenier’s extra-sec product), and is now an extra-sec Curaçao liqueur.
Watch for a future post giving the grams of sugar per liter for each easily-obtained true Curaçao liqueur still on the market!
elementalmixology.blog 
Can you source your historical facts? It’s difficult to reconcile them with, say, this post:
Which post are you referring to? I don’t see a link.
Well, in the meantime, I updated the post itself to include sources for what seemed to me to be the most obvious points.
I have given historic sources in the following post:
https://elementalmixology.wordpress.com/2014/02/07/curacao-liqueur-and-triple-sec-curacao-liqueur-old-texts-and-current-shopping-sources/
Sorry. I didn’t realize the comments were in HTML. I put the link in between < and > and it got mangled. Here it is: http://www.alcademics.com/2011/02/whats-the-difference-between-orange-curacao-and-triple-sec-.html
Well, that post doesn’t give any sources. I think that the French meaning of “triple sec” is very clear: triple-dry. Some triple-sec Curaçao liqueurs are also triple-distilled, and some bear the words “triple-distllée” in addition. My guess is that some of the confusion may have come from that. All Curaçao liqueurs are distilled – otherwise the bitterness of the orange peel would be too overwhelming if the simple maceration were bottled, even after sweetening. I believe that the Netherlanders may have used grape spirits for the maceration. I find this is unlikely, though, since the Germanic tradition for the production of geists, or secondary spirits such as genever and its ancestor, wacholderbeerengeist, usually involves maceration in grain spirits of botanical material. Genever is still often made using grain spirits by the Netherlanders. But, even if they did use grape spirits, it definitely didn’t come out of the still after the orange maceration with anything like the character of an aged brandy that is woody and brown. Also, all good Curaçao liqueurs, triple-sec or not, are even today secondarily distilled in pot stills The continuous still would remove too much of the orange flavor.
I decided to research this liqueur after encountering it at a bar here in Seattle. You seem awesomely cantankerous and well-informed. I shall be a regular reader henceforth.