The company Eduard Kettner was founded in 1884, in Köln.
After WWII, they evolved into one of the leading mail order companies for hunting goods in Germany. Since around 2000, they went through a couple of owners and insolvencies.
The name still exists, but only for clothing goods.

I still remember the catalogs from the 60s and 70s. Rifles with M98 action were always offered, from cheap military conversions to elaborate custom quality work.
Most of them probably bought in from whatever workshops all over Europe.

So, it's a trader, not a gunmaker's shop - impossible to compare to good old names such as Sauer or Merkel.

There was also the company Franz Kettner, a brother of Eduard. But this went out of business around WWII or even earlier.

fuhrmann