CCW-MFG's Glock 42 Magazine Extension adds +2 rounds of 380 ACP to your factory magazine.  This CCW-MEG42 extension has a fit/feel/quality one should expect following the lines of their factory firearm.  The extension is easily installed and disassembled with one screw.  
This extension is manufactured in the USA from 6061-T6 aircraft grade aluminum, and hard black anodized.