Need a little help with 1937 S/42

Hi everyone, I'm new to both this forum and Lugers in general, so please excuse the overwhelming stench of "novice" coming from your computer. I spent some time yesterday examining an S/42 in pretty good shape. The guy told me it was made in 1937. I do know that '42' is the code for Mauser-Werke AG, Oberndorf on the Neckar, and was later replaced by the 'byf' code, but what does the 'S' stand for and what is the correct letter code for 1937? I would appreciate any and all info (or links) concerning S/42-marked Lugers as it may become the first one I buy. Does $425 soung like too much? It was refinished professionally and has all matching numbers, good grips, shiny bore and deep rifling. I want to start with something I can spend quality time with on the firing line. Thanks in advance for any help you may have. My profile and bad photo are up for viewing, thanks for letting me be a part of this group. Take care....Denny