I really have nothing solid but it reminds me of a Sauer I once owned. Could it be a JP Sauer, unmarked, made for a non-continental market? Now that's a wild guess
There's likely some proof marks, maybe on the barrels' underside. Someone familiar with proof marks might be able to shed some light on manufacturer. The proof mark may or may not reflect exact manufacturer but certainly would indicate country of origin. Here's a list that could possibly be used to match to the gun.
http://www.nramuseum.org/media/940944/proofmarks.pdf
The shot gun is made in Belgium , four five eight . It was stamped " 12 Bore DBBL Made In Belgium " when it's previous owner brought it into India through customs prior to 1959 . Before 1972 , when any resident would purchase a fire arm from a foreign country and bring it back to India for private use , the customs officials would always ( have someone) stamp the calibre and make of the gun , as well as the country of the gun's manufacture on the underside of the gun.
It has 3 inch chambers , extractors and 28 inch barrels. The left barrel is fully choked. The right barrel is modified choked. I have taken extremely good care of the shot gun since 1959 and it has provided me with 60 + years of flawless service. Loaded with Eley Alphamax 3 inch cartridges , which were hand loaded with spherical ball bullets , l used it for all of my personal hunting as well to back clients up for shikar. The grand old gun helped me slay 20 panthers ( several of them , man eaters ) 12 royal Bengal tigers ( 11 of which were wounded by my clients and had charged our shikar party . ) , gaurs , water buffalo , wild boar , Asian Sloth Bear , crocodiles and countless game for the pot . It has never let me down 1 single time .
The gun got me out of many a sticky situation.
The forum member , Red Leg was kind enough to help me identify my gun . He said that the action is similar to Auguste Francotte of Belgium and that , since my gun is unmarked , it is what collectors know , as a " Guild Gun " that is made by an obscure maker of the gun maker's guild in Liege , Belgium .
This shot gun is probably 1 of my most valued possessions . I have kept it in mint condition all these years and l use it through out the year for all my hunting and skeet shooting purposes .
I often write articles in the " articles " section of African Hunting forums about my old days as a professional shikaree in Nagpur , India and the gun receives a great deal of mention in every story .
Perhaps , some of them may interest you .
I even dedicated 1 article , just to that old Belgian shot gun .
https://www.africahunting.com/threads/a-gun-which-lasted-a-career-a-life-time.54231/