HELP: Belgium double barrel SxS

I need some help trying to find who built this gun and when. The only identifying marks are under the forearm. Acier Martele, from that I was able to determine it was Belgium steel. It was also stamped 12/70, from that I was able find the gauge and chamber length. There are a couple of other proof marks but that is it. What else am I looking for and where should I be looking? I dont care if its a 10 or 10,000 dollar gun as its not going anywhere but just trying to research it for personal knowledge.