This from Oldgun.net:
Robert- Franz Kettner operated in Suhl, circa 1891-1939, and an Edward Kettner operated in Cologne (Koln) circa 1890 and later, so perhaps the two are related or merged at some point under Franz's name. There is not yet any good reference material on the many fine German gunsmiths who operated prior to WW2, although we have heard someone is working on such a book. If your rifle has hammers, I would put it toward the earlier part of this time spread, and if hammerless, toward the later end. If it has crown over U proofmarks it is pre-1934 and if Nazi eagle proofs, then later.