A common theory is that the 'S' code, apart from being a year letter code for 1936 that was not officially introduced, was also part of the scheme to hide the fact that other companies than the ones that were officially allowed to manufacture were also producing arms and ammunition. In the 1930s Simson was still the only 'official' supplier and it is thought that the 'S' is an attempt to fool the outside world into thinking that Simson was involved. 42 was used to identify the actual maker, in this case Mauser.
A similar marking sequence can be found on ammunition, where Polte in Magdeburg was the only official supplier and other companies were given a 'P' code, followed by a number as well.
The sequence they used worked well, we are still confused at times
Alvin: I can suggest the book by Michael Heidler on the markings. The book not only shows an extensive codes list, but also a lot about the coding systems themselves, including interesting copies of original 1920s and 1930s documents. 'Deutsche Fertigungskennzeichen bis 1945'.
It can be ordered from Michael Heidler directly at: Michael Heidler [codebuch@rocketmail.com]