The MP28,II was Hugo Schmeisser’s improved take on the original World War One MP18,I design. It used a simple box magazine in place of the Luger drum magazines, and this magazine would form the basis for a long series of military SMG magazines. It was a double-stack, single feed design because Schmeisser thought this would prevent some malfunctions that were possible with double-feed magazines (and because Mauser probably had a patent on the double feed box magazine at the time). This magazine would be used in conversions of MP18 guns, and would also be the model for the MP-38/40 subsequent British Sten gun magazines.
The MP28 also introduced a semiautomatic selector switch, where the MP18 had been a fully automatic only design. It is the presence of this selector button over the trigger, along with a tangent sight instead of a simple flip-up notch that can be used to distinguish between and updated MP18 and an MP28.
While the MP28 was not formally adopted by the German military, it was used by police and SS units, as well as being adopted or copied by a wide selection of other nations, including Portugal, Spain, China, Japan, and Ethiopia.