Well, you've got me stumped. Looks like there were a bunch of Dumoulins who made, or are making shotguns......but then, I'm not an expert on Belgian shotguns.
I'd suggest that you go over to Shotgunworld Forum and ask the experts there.....they're really shotgun specialists and know their stuff.
One saving grace.....all of the "Dumoulins" I have any info on are, or were, high quality shotgun makers, so you probably don't have a low end "Hardward Store" shotgun.
Sorry I couldn't have been more help.