Yeah, let's see the flats. From the images it appears that it was proofed in Zella-Mehlis post 1911 and with the Louis Kelber effort on the imported Krupp steel tubes, I would guess that it was made by one of the Schillings from the Zella-Mehlis area. Is the Nitro stamp in script? There should be a date stamp as well as a ledger number on the flats. Also there will be a diameter stamp in mm. Otto Bock was the firearms merchant/retailer.

Kind Regards,

Raimey
rse