In a report on the history of Solothurn Waffenfabrik I have found this drawing of a 25.4x148B cartridge which was described as having been for the S19-100 gun.
As the caliber is exactly 1" I wonder if the caliber was a requirement of a potential customer maybe. Here in particular Argentina as they are known to have adopted a 1" Vickers AA gun (for their navy). Maybe the S19-100 was a competitor?
Is there more info on the gun or the ammunition? Any photos available?