Hans and Helmut Zehner were gunsmiths from a family that dates back to at least 1790. Found and ad from them in an antique bookshop in Micanopy Florida about 30 years ago. They had a gunshop in Frankfurt Germany in the 1950's. We were stationed there during that time frame. My Father had them make him a Belgian FN Mauser in 30.06. It is similarly engraved with a Hersch scene instead of the Boar pictured on your gun. There is extensive wood and metal engraving coverage to about 75-80% of both. Additionally, it is fitted with a Hensoldt Wetzlar 8x 52 claw mounted scope with a German 3 post reticle.
This rifle and my Mom's Sako .222 Remington L46 ( Also with claw mounted H&W 3 post scope) were used by them on a number of nightime full moon drive hunts courtesy of the local forester (Herr Klempau) with whom my father had become very good friends with in the postwar years. I still have the Sako while my brother has the FN. Both are very accurate shooters to this day.
The FN was appraised at $15,000.00 in 1987, mainly due to the fact that they are in a class with the highest of British and European limited production guns. Think H&H, Westley Richards and the like. I have been told that the guns made by Zehner in that time frame are essentially one-offs built for specific customers as was the case with my Father's FN.
I have no knowledge of the production numbers they built during that time frame, nor anything about the relationship they had with Steyr in that time frame.
At any rate, were that gun in my collection, I would take the time and effort to do some research and GET A PROFESSIONAL APPRAISAL before ever considering selling it. It is very possible you have a much more valuable gun in your possession than you think. I just did a Bing search on them and it found quite a number of links.
If it turns out to be a diamond, I want a cut of the proceeds
ETA: talked with my Mom to correct the third sentence in the first paragraph.