Dear Bill,
I don't know when Mauser started using the Original Mauser Banner but for sure in 1937/1938 there are some advertising where the Original Banner Mauser is used.
Concerning the use of this banner on the Post WWII production I show you something:
The following picture shows two 29/70 models. On the left the 19th pistol made (10.00.1019) that still has some prototype features.
Among them the Mauser Banner. In fact if you compare the Mauser Banner with the one on the right (Mauser 29/70 cut away) you can see the difference between the Mauser Banner used for the prototype and the real early production and the standard one.
Now the one used for the prototype/Early production is the same used in the pre-WWII commercial advertising. Then Mauser, for any reasons, change it with the one on the right.
Hafe fun
Mauro