The +2 magazine extension is manufactured by Taylor Freelance, and is optimized for use with the 10 round magazine.  The newest line of extensions from TF have lightening cuts which reduced the weight by ~1/2 oz and they look badass! 

Features

• Machined 6061 Aluminium
• Harcoat Anodized MIL-A-8625F Type III / Color as Shown
• Thumbscrew and gasket retention lets you choose-- finger tight for quick on-off or screwdriver tight for maximum security. 
• No need to swap springs, the TF +2 extension utilizes the spring from the host magazine. 

Notice

The TF extensions were designed to work primarily with the GEN-12 10 round magazines. The extension will work on the 5 round magazine, but they were not intended to be, and feeding is not guaranteed on the 5 round magazines.