1939 s/42 ????

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Hello and thank you for helping me with my Luger questions.

I have a 1939 S/42 that is all matching numbers except for the magazine. The bluing looks extremely good for a 70 year old gun and I would like to know:

1. Where and how many were made? (is it a rare one)
2. What branch of the military used it?
3. What is the value?

My wife's Grandfather brought it back from the war along with a neat little desk clock. I believe he said he "acquired" both from a German Officer.

I'll post a pic of the clock if you are interested because it has the German Reich stamped inside.

Thanks in advance for any info.