Definitely 9x57 chambering available; contemporarily nomenclature, simply "9mm". Also notably only available in the early model featuring schnabel stock & non-ramp front sight, 1928-31 era. Such if your friend seeks pseudo-authenticity! Conversely, better appearance & metallurgy in boring out such as a common .270 Win barrel from the later NRA edition! The difference between early - nickel steel and later "Winchester Proof Steel, aka - chrome moly steel!
Just my take!
Best & good luck to him!
John
