Brother1, what a fascinating gun you have found! The barrel treatment is unique and the Damascus very attractive. Congratulations are due.
I tend to agree that the gun is Austrian in origin, but cannot prove it. While the gun bears Belgian markings, they are not proof marks but inspector's marks. The gun underwent no proof testing in Belgium, but was stamped on import, after inspection. (I once had a Spanish AyA so marked).
I think the gun is from later than 1880. The German system of proof marking was instituted in 1891, and the Austrians followed suite, but were a little more lax in the application of the law as it pertained to guns for export. If it is a German gun then it is certainly earlier than 1891, but if (as suspected) it is Austrian it could be later. The reason that I think it could be a little later is that it is equipped with an Anson fore end. The Anson patent did not expire until 1892 (I think) and your gun shows no patent markings. In addition the bar action sidelocks were less popular among the continental guns than back action designs until the late 1880's. I would place your gun as being from the early 1890's.
If I were you I would send your pictures to Martin Hagn (info@martiniandhagngunmakers.com). He is of German origin and studied gunmaking in Ferlach, so he would be in a good position to both identify the piece and assign a value.
Truly an interesting gun, thanks for sharing.
Sharptail
