This is the Mec-Gar 16 round .40 S&W magazine for Tanfoglio Witness Large Frame pistols.
Mec-Gar are the industry leader in pistol magazine production, providing OEM manufacturing for many big names including CZ, Beretta and Browning.
Frame Size
Tanfoglio have two frame sizes. You can ID them from the magazine width.
This is the Large Frame size - magazines are approx 21.4mm wide at the rear of the magazine.