Welcome Michiel,
You covered pretty much all the possibilities, It is my understanding that these masks were used after the war, and that they were made from surrendered German masks that were converted to use the French filter, I have not seen one in a German can, but this is possible (it could have been done by a collector who did not realize it was not German?)
The problem is that there is very little written information from the period about these masks, so we are left to use our best guess and keep looking for more information. A period photo would be of great help, but photos of Belgian troops are rare.
Short answer, No I do not think anyone knows the right story.
Best wishes, and welcome again
Gus