Raimey,
The Kettner has the look of a Meffert. Kettner mostly dealt in guns from other manufacturers. I suspect the 517 is not a ga. designation, this wouldn't fit with the case length(35mm),nor time period (well after 1912).It could instead be the bore dia.in mm, which would admittedly be very tight( 5.17 mm-0.2035", more common 5.3mm-0.2085")The 5.6x35 Vierling is sometimes encountered with very tight barrel.Not having the gun "in hand", I wouldn't bet the farm on it though.
Mike