This German single shot was made by the Römerwerk, Suhl. It was proofed in Suhl 1/28 = January 1928 for using 12 2 ½” shells. 13/1 was the gauge number for a tight 12 bore. Just beside the “eagle” mark for first proof you may see the proofmark for “rifled choke” (crown over intertwined SW) I mentioned above.
BTW, straight rifling or straight rifled chokes were not meant for shooting ball all the time. Instead, it was more often used as a means to improve shot patterns. The theory is: The straight rifling prevents shot charge and wadding from rotating willy nilly while passing the barrel. This is thought to reduce centrifugal dispersion of the shot charge after leaving the muzzle. Apparently a brilliant idea reinvented by each generation for more than 200 years. The earliest example I have seen was a flintlock fowler from about 1790. Zimmer, 1872, mentions it as an old but bum idea. Then I have seen some 1890s Austrian - Bohemian shotguns with straight rifling. I found it last mentioned reinvented as a “new improvement” in a 1960s Gun Digest. An American then attached choke tubes with straight rifling to his shotgun and claimed vastly improved patterns as did all the other “inventors” before.