I have trouble opening your pictures, so the following is something of a guess.As far as I know, Browning never sold separate double rifle B25 receivers (those with the solid bottom and thicker side-walls) to gunmakers. Because of the fame of the B25 Superposé , B25 receivers taken from shotguns were always popular in Belgium for building double rifles. I know that Pletsers, a Belgium custom maker and specialist of making big bores on Browning receivers, prefers to use receivers taken from B25 trap-guns. Apparantly these receivers were machined with extra-thick side-walls and thus stronger than B25 receivers used on game guns. Possibly Dumoulin follows this practise also.
Greetings,
Livinus