This is the 5 round Steyr SSG factory rotary magazine in .243 for early model Model L series rifles and SSGs with trigger guard magazine releases.
This magazine does not have the side squeeze release grips commonly seen on later models and will only fit the old format SSG rifles with trigger guard mounted release levers.
The rotary magazine design is flush fit to the stock and is made of a tough polymer with a clear viewing plate at the rear.