Jose
Have you actually tried to take the barrels off? I had an old Belgium gun very similar to yours, (only not nearly as nice!). On mine there was a swing lever, underneath the opening lever, that allowed removal of the barrels. I see such a lever in the photo you presented.
Has this gun been restored? The gun looks very nice. The one I had was definitly a JABC, but yours looks to be a higher grade gun then mine.
I've attached a picture of my JABC
There is another swing lever towards the front underneath the end of the locking lever. You swing that lever to the side and it unlocks the barrels. I don't have the gun any more or I'd take a picture of it with the barrels taken off.
Steve