SIG began making SMGs in the 1920s with a licensed copy of the Bergmann. In 1927 the license expired, and they began working on their own designs, the first of which was introduced as the Modell 1930. This was replaced in 1933 by a design from SIG engineer Gotthard End, using Pal Kiraly’s lever-delayed blowback mechanism. This was the MKMO or MKPO, with the military (M) long barrel or police (P) short barrel. It was chambered for 9x25mm Mauser, and was made to beautiful, exacting standards as one would expect of 1930s SIG. However, this made it expensive, and the 9x25mm cartridge never really caught on. In 1937 the design was simplified to the MKMS or MKPS, now using a simple blowback mechanism and offered in 7.63mm Mauser, 7.65mm Parabellum, and 9x19mm Parabellum. The example we have today is a 9x19mm MKPS.
Even in this less expensive form, the guns were really not successful. Only 1,228 were made of all models combined, with the most notable buyers being the Vatican Swiss Guard and Finland (who bought 282 in March-June 1940). Mechanically, they have several excellent features, including folding magazine wells for storage and several integral mechanical safeties. The trigger will not function if the receiver endcap is not fully screwed on, and it cannot be unscrewed unless the manual safety is engaged. They do not have a semiauto function, however.