Marc,
Thank you for taking pictures, and letting us share them.
Let us start with the easy thing: The intertwined W and R stands for Werks Revision meaning: factory control. This stamp was used from mid-1936 to the end of the Luger pistol production in 1942. It replaced most (not all) of the various letters, signs and characters that were stamped by workers mostly inside the pistol on the frame, receiver, and toggle train. Mauser's system of quality control consisted of the extended use of factory gauges. Every part, even the axles and the small springs, were checked on a precise measurement. In Oberndorf there was a hall with sixty large tables where all parts were checked by sixty people - mostly women.
The larger parts like the frame and the receiver, demanded 150+ different toolings before they were ready for the pistol assembly line. If one of these toolings would not have been 100% succesful, the frame would be worthless at the end. Therefore the measurements were checked everytime when some 30 toolings had passed. The worker who checked this had a personal stamp. An -A-, a -b-, a -+-, a -2-, these were symbols which could be linked later to the person who was responsible for that specific quality check.
Mauser replaced this old DWM sytem in 1936 for the quality checks on the receiver and the toggle train. The R+W stamp was used, while the number of at random symbols went down sharply. The R+W stamps are found under the front and rear toggle link, and mostly also under the receiver on the left rail. The quality check procedure of the frame did not change.
Now about your 42 banner serial number xxxxg.
It is a Luger pistol that was inspected and accepted by the German Army inspector 135 (code for Major Rosenhagen). The 42 date on the chamber, and the suffix -g- of the serial number, indicate a production in the month of May 1942.
At that time, Mauser was using obsolete replacement parts that were mixed into the production of the military pistols. We see chamber - toggle combinations like: 1940-byf, 42-42, 41-S/42, and, indeed also some banners. It seems that the Army Waffenamt in Berlin had no objections to Mauser's cleaning up party. An estimated total of some 4,500 "out-of-sequence"- pistols are found between 41-byf suffix -x-, and 42-byf suffix -k-.
So this Luger pistol has not been changed later - it came like this from Mauser's production line. It is a pity, however, that it has been reblued.
There are a few more military banners reported in the -g- block.
I would like to have the serial number of your pistol -for my database and for the statistics of the book I am now finishing.
I hope this helps.