Unusual cartridge - Police Gun by Greener - I found at a gun show about 30 years ago...

The bottle necked Mark III Special Greener Police Gun Shotshell. Case of drawn brass, crimped closure (10 point folded or roll crimp). The cartridge was specially designed for use with Greener’s Police Gun Mark III and deliberately intended to be used with nothing else. The Police Gun was a Martini-Action shotgun used by British Colonial Police forces (particularly in Egypt) during the early 1900’s. It was adapted so that it would only allow cartridges, made specifically for the weapon, to be fired.