My first assumption, on seeing the trigger guard missing, was that the guard was of horn. It would match the buttplate, which also appears to be of horn.
Just to be a little clearer: there are two sets of "flats", which are the flat surfaces where the receiver (the part with the shoulder stock attached) and the barrels (not their business end) come together when the gun is closed. The "flats" on the receiver are called the "watertable" and the flats on the barrel are the "barrel flats".
Most of the proof marks we're interested in, and which will do a lot to flesh out the provenance, identity (and worth) of your gun will be located on or around the barrel flats. As noted above, the identities of the actual workmen who made the tubes (barrels), joined them (knitters), inspectors and the shops they worked for can be known because they all had unique identifying marks. (Some of the guys here have done some real scholarship on this.) Similarly, with German guns it is often possible to know the exact month in which the gun was "born" (sometimes, they take a while to get "made") because that is stamped on the barrel flats; it's possible this gun was made before that became the law in Germany, but we'll see when you get more pictures up. (From the looks of the design, it looks to me like an interwar gun.) And, of course, the caliber of the rounds used in this gun will be revealed by the marks.
As a double rifle, this gun is likely to be worth more than a similar double-barrelled shotgun made by the same shops/people at the same time and with the same level of engraving, wood and finishing. This is for two basic reasons: there were many fewer double rifles made than double shotguns and double rifles were far more expensive to begin with. Double rifles were (probably most) commonly used for dangerous game (big cats, rhinos, buffalo, elephants, etc.) because they provided the hunter with a second shot instantly ready without having to work the action in case the first one didn't do the job. Because the average guy didn't have the means or time to go the far distance and expense of safari, the average guy didn't have need for a double rifle.
All that said: I really like the engraving on the receiver. Looks good. Your grandfather had good taste in guns.
Last edited by Dave in Maine; 01/10/12 03:28 PM.