First of all, my caption to that picture should read 1889, not 1899; by 1901, Curacao Marnier was calling itself Grand Marnier (perhaps as a result of winning all those medals). And yeah, this pretty much proves that Grand Marnier is to orange curacao as Cointreau is to white or triple-sec curacao.
As for the teminology. Unfortunately, the history of curacao is a sort of third rail for the would-be drink historian; I've found it so, anyway--as soon as you think you've got something figured out, something else comes up to prove you wrong.
Case in point, that extract from Duplais. This could very well explain the "triple orange" in the poster. But then there's this, from Artaud de Montor's 1837 Encyclopédie des gens du monde & c.
…les eaux distilleés ont été divisées en odorantes et non-odorantes, et l’on a remarqué que leur vertu dépendait en grand partie de la manière dont la distillation avait été conduite. Lorsqu’on veut les avoir parfaits, il faut faire passer plusiers fois la meme eau sur de nouvelles plantes: c’est ce qu’on nomme eaux distillés doubles, triples.
A rough translation:
Distilled spirits have been classified as fragrant or non-fragrant, and it has been remarked that their virtue depends in large part on the manner in which the distillation has been performed. Should one wish them to be perfect, one must pass the same distillate several times over new botanicals; this makes for what are kown as "double" spirits, "triple" spirits etc.
So--a "triple" curacao is one that has been distilled three times, with a fresh batch of orange peel used in each distillation. All well and good, but by the time Curacao Marnier and Cointreau were on the market, pot-still distillation (a batch process) was being replaced by the continuous column-still process, which makes those three separate distillations obsolete.
Then again, as far as I can tell these terms weren't regulated, so it could be mere empty verbiage, stating in effect merely that it's a high-quality product with a concentrated orange flavor. But I'm not sure what to make of the passage from Duplais, or how Grand Marnier's cognac base figures into things.
P.S. I think this last factor explains the two MB products: most curacaos use a neutral spirit base, not a brandy base like the GM Cordon Rouge (indeed, there used to be--and maybe still is--a cheaper, neutral spirit-based Grand Marnier, the "Cordon Jaune," or Yellow Ribbon). Thus two grades of orange curacao, with an MB for each.