This is the flush fitting 4 round .22WMR factory magazine for the Anschutz 1516 series (1516-U4 012960).
Made from high quality steel it is also strippable for cleaning. It can also be used with .17HMR ammunition if you prefer a magazine without the front feed ramp.
Anschutz currently don't make a higher capacity magazine for the .22WMR so most people carry a few of these. It is possible to get them to take five rounds by filing the follower leg down slightly.
Please note this is the current version of the magazine and may not be compatible with very early .22WMR models.