I would recommend to make a closer lock on the buttplate with a good magnifying glass. if carved you must see some traces where the tool have cut the material. if pressed under heat the surfaced will look like some kind of glaze, maybe with little bubble's where it was become to hot.
for the history of your rifle: in the "god old day" a double rifle like yours was very expensive. chance is very high that it was a noble man from upper saxony, a Herr von XYZ. such a big bore was bought for shooting red dear and wild boar. some older gentleman like this here in the middle

btw, on this site from the member chapmen is an old gründig paper
http://www.jagdwaffensammler.de/index.php?id=12