It is not Swiss, but British.
“ST” is Royal Ordnance Factory Steeton, made in 1944.
TonyE is of course correct about the manufacturer.
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.
44 is the date (1944), but I’m sure you already figured that one out.
Thanks, I assume “Z” is for nitrocellulose. How common are these in the UK, somehow I don’t see them in the States? I see Falcon beat me to the “Z”, it actually stands for “Zombie” (Halloween humour).
These are very common in the UK. You will see them at nearly all military collectables shows. Dealers usually want £2 or £3 each for them.