The cover of the lockcase, although rivetted, is not difficult to remove and replace. The catalogue shows the face of the trunnnon as concave, the photo appears to be flat.
This catalog entry is intriguing. It offers 12 differs as regular, but 7770 maximum possible - from 2 levers! Even offers 1200 mastered. No wonder there is extra cost for the non-regular differs; how are they obtained (assuming still made with 2 levers)?