The Marjacq is French and practically the same as Opinel.
Holy hell,
DO NOT google Marjacq images without the Safe Search on. I don't know what the heck Marjacq means in French, but it'll bring up images that you don't want to see, and you don't want your family, employers, or the FBI know that you've seen. 'Nuff said?
As for the knife, I don't know if that's the knife I was thinking of, but the Opinel certainly looks more like Dumbledore's knife than the Marjacq, at least with the blade profile. Do Opinels have the engraved/stamped design on the reverse side of the blade that's on Dumbledores? Every dang picture of an Opinel that google brings up shows it from the obverse.
It definitely looks like the movie prop department took an Opinel and modified it, or at least used its design as a template. Prop makers are extremely talented people, and it's possible they fabricated the prop from scratch. It's also possible that the prop itself is
not an functioning folding knife (or knife-like-object). I loaded up the DVD (yeah, yeah, I know, I'm a dork) and you only see him "pull" the knife, not open it or close it. The blade definitely looks Opinel-shaped. You don't get a good view of the rest of the knife though.
In the book (yeah, yeah, I know...) it's simply described as a "short silver knife of the kind Harry used to chop potion ingredients", which earlier is described as being a silver bladed dagger. That apparently went out the window when the movie adaptation was made, but who really cares?
I have no doubt that the replica prop is a faithful reproduction of the actual prop, but it's definitely an interesting question of how the original prop came about.