It seems this Buechsflinte was chambered for a 20ga x50mm (about 2") ball cartridge. Such cartridge cases were available in various gauges and lengths, for loading by a local Gunshop or maybe the guns owner. I think old catalogs showed these cases under the "Express" heading. The confusion is exacerbated by the gun having been made before the first proof law was in effect. If it had been proofed, the markings would show whether a barrel was proofed for shot (crown S) or a single projectile (bullet) (crown G).
Mike
