Developed by Heinrich Vollmer in the 1920s, the EMP was the end result of many years of experimentation by Vollmer with the clandestine support of the Weimar German Reichswehr; once that support ended, Vollmer sold out his designs to Erma Werke, who marketed the gun under the "EMP" (Erma Maschinenpistole). Under Erma's banner, the EMP saw action on both sides of the Spanish Civil War, was exported to Mexico and Yugoslavia, and saw domestic use with police formations and the SS. Patterned after the MP28, Vollmer's design is typical of the "transitional" SMGs between the heavy metal guns of WWI and the tube guns of WWII, using a tubular receiver with a hardwood stock. One critical feature introduced by Vollmer was a captive telescoping recoil spring assembly; later applied to the MP38 and MP40, this captive assembly kept the spring fully contained and protected from damage, easing maintenance and handling for troops in the field. Blade front and two-leaf rear sights, with the "eagle/N" proof and "MP" on the receiver socket, and matching serial numbers on the socket, magazine well, barrel shroud and bolt. A two-position "F" and "S" marked safety lever locks the bolt either fully open or closed when engaged, with a "E" and "D" (semi/full) selector switch on the right side of the trigger guard, and a takedown lever at the rear of the trigger guard. The selector switch works in conjunction with a two-stage progressive trigger; "E" restricts the trigger to semi-auto only, "D" permits semi-auto fire with a light squeeze and full auto with a harder pull. Smooth pistol grip stock with a vertical hardwood foregrip, 98k style sling cut, and a smooth steel buttplate.