I just received a "plus one" extension for 20 round Promag. It is a CZ-UB manufactured item imported by Armory Craft. It offers ample space in the gauge, about 3mm short, looks good and easy to install, uses a set screw to keep it from sliding off. I am getting 27 in which should be reloadable as it will take about 27.5 rounds. I tried a "plus two" which required milling several mm off the bottom which destroyed its function. Promag about $25, Grams spring and follower about $18, Basepad about $32, = $75.