This is the factory Beretta PX4 Storm 20-round extended magazine in 9mm. 

It is also compatible with both Compact and Sub-Compact PX4 Storm pistols, but will extend below the base of the grip.

If you are purchasing this magazine for a CX4 Carbine, please ensure that your pistol has a compatible adapter. If you are unsure, you can check it online by:

• Visiting www.berettasupport.com.
• Choose “Technical Data Search” on the right side of the screen.
• Enter your serial number.
• The resulting data will include whether or not your CX4 is fitted with a 92FS or PX4 adapter (ie Product Description: CX4 STORM (PX4) 9MM/17RD).