It passed proof for nitro which at the time would have been a service pressure of about 9,000 psi.
When it passed proof, it the barrels weighed 1.567 Kgs or 3.45 lbs. If the barrels weigh less than this when you weigh them today, some one has messed with them.
The barrels have the trade mark of Delcour-Dupont.
Delcour-Dupont was a barrel maker in Nessonvaux, Belgium. Nessonvaux is short drive outside of Liege. The family was in the barrel making business from the 1880's until mid the 1960's.
This is Jean-Baptiste Delcour-Dupont and his son Oscar, 1924. They are standing outside their factory. The stones in the back ground were used to grind damascus barrels. Their shop continued make damascus until 1938.
The barrels on your gun appear to be fluid steel. But that really does not matter, whether fluid or damascus they all had to pass proof, which was for less than our current pressures. Also we are talking about a 90 year old gun here, so the stock should not be submitted to heavy stress. You can find suitable ammunition for older shotguns from some one like RST, http://www.rstshells.com
You can view Belgian proof marks here:
You can view Belgian trade marks here:
I do not see anything that indicates the maker.