The original cartridge for this type gun should be regular 11.43x23mm. Why did new Shansei have a 11.43x26mm chamber being a myth. Probably it's upon importer's specification, creating a chamber that no production cartridge can be fired from it -- it's marketed as a collectible, but usually people's curiosity is suppressed by cost, not by words. If many buyers ignore the warning and fire their instances, and a few were injured, the importer will be in trouble... so importer made the chamber longer. To fire it, buyer must modify the gun. If the gun is modified and it hurt the operator in range, that's not importer's responsibility anymore
Just my guess on this weird chamber.