I think you are right about the 9.3X57.
Fraser often had his own loads for a particular cartridge loaded with his patent ratchet bullet which is nothing more than a jacketed bullet with angled slits cut into it.
Of course, Mike is right. A chamber cast will clear that up easily and being a takedown, there is not a simpler chamber to take a cast of. If you don't have any cerrosafe, use a hot glue gun or even wax for the cast - that will give you enough information for an ID.