This is an S&W CSX 9mm 50-round drum magazine, manufactured by Promag. 

• 50-round drum magazine
• Fits Smith & Wesson CSX 9mm pistols
• Magazine body constructed of a proprietary DuPont Zytel-based polymer
• Blue steel upper magazine tower
• Injection-molded magazine follower
• Magazine spring formed from heat-treated stainless steel
• Made in USA