What a lovely old treasure you have there brcampbe!
If one of our German members see this they'll be able to guide you through the maze of proof marks & hopefully there may still be a cartouche somewhere that proves he did the work!
Herr Brenneke did bring out some wonderful cartridges.
This one could have been in 8x64mm as the 7x64mm came out in 1917 or thereabouts.
If the barrel is shot out then it may be possible to return it back to that calibre by reboring?
Family heirloom that one!