View source for German Mauser S42 Code G Date Luger P.08 Pistol

The s/42 G Code Mauser P08 Luger, manufactured by Mauser in 1935 for the German Government and is a semi-automatic toggle-locked pistol with eight round detachable magazine.  It is chambered for the 9x19mm (9mm Luger) (9mm Parabellum) cartridge. In 1930, Mauser took over the manufacture of P.08 from DWM and thus made the S42 Code G Date Lugers.

The Treaty of Versailles only allowed the German Simson & Company to produce any of the few small arms granted Germany after World War I, arms production was done secretly at Mauser, among others.
When Germany began to rearm itself in the 1930s, it used these letters instead of numbers to annotate the year of production of Lugers.
The “S” stands for “Simson” to throw off suspicion, and “/42” was the rest of the code for the Mauser factory. 
Dates were left off of the barrel until about 1936 when Germany’s arms production and military activities started to be brought into the light.  

In 1934 Mauser got its first Military P.08 contract from the German government.  These pistols were marked with "K."  Beginning in 1935 the Mauser factory began by marking the guns with the "G" instead of "K."  

The barrel extension is marked “G” on the chamber ring (code for 1935 production), the left is marked with the serial number.  Typically the extractor is marked  with the last 2 digits of the serial number, found again on the trigger, sideplate, takedown lever, safety, bolt stop, the left of the firing pin, breech-block, top of the front toggle link, rear of the back toggle link, and the sear.  The top is marked “S/42” (Mauser production code).

The German Mauser S42 Code G Date Luger were finished with straw bluing and yellow straw bluing of certain small components. Later on the Mauser factory stopped straw bluing small components on its Lugers in 1937 to meet higher production demands. They phased out the rust blue process and "straw finishing" small parts and levers on the P08, choosing to salt blue all parts of the weapon at one time.