This particular 20mm casing was manufactured in Royal Ordnance Factory Radway Green, Radway Green, UK The “OeZ” marking means “Oerlikon, Loaded with Nitrocellulose powder”. This maring was used to differentiate the semi-rimmed Oerlikon rounds with the similar looking (but not interchangeable) 20m Hispano. In British service, a “Z” stamped on the base of a case 20mm or below in calibre indicates that the case is loaded with nitrocellulose flake powder. This is opposed to the typical British Cordite “spaghetti” rod type propellant.
Naval Artillery
Oerlikon 20 mm cannon, Military Wiki
WW2 1942 US Dated 20mm Ammunition Shell Casing
WW2 BRITISH FIELD GEAR & WEAPONS
20 MM Question - General Ammunition Discussion - International
Anti-aircraft installations created on the basis of 20-30-mm
Oerlikon 20 mm cannon - Wikipedia
Is an anti-aircraft gun just a powerful gun with big bullets? - Quora
View 460 Ammunition for Sale in North West- Gunstar