In order to circumvent Versailles Treaty restrictions on arms manufacture, the German Rheinmetall firm purchased a small Swiss company called Solothurn Waffenfabrik in 1929, allowing it to route its business through Switzerland instead of Germany. One of its first products was the S1-100 submachine gun, designed by Louis Stange. This was an excellent example of a first generation submachine gun, made to very high standards with an elaborately milled receiver. They were expensive, premium guns in the 1930s, and were sold worldwide, including contracts to Bolivia, El Salvador, Japan, Thailand, Uruguay, Portugal, Austria, and more. To accommodate these different client militaries, the gun was offered in 9mm Steyr, 7.63mm Mauser, 7.65 Parabellum, and .45 ACP.
This particular example appears to be a Japanese one, purchase in the mid 1930s to equip some elite Japanese units like the Special Naval Landing Force. It is in 7.62mm Mauser and shows the D/E selector markings one would expect from that contract (other contracts used markings with Spanish or Portuguese abbreviations). This gun would have been captured by an American serviceman during World War II and brought back, to be registered later in the 1968 amnesty. It is all original, with the exception of what appears to be a replaced barrel.