confused... Father in laws Luger

[Wicked Klown]

Last night my father in law gave me his Luger to take to the range today. His dad got it when over seas during WWII, he was part of the Omaha beach landing. Well I came across this site last night when looking up the gun. Have to say so far I love and hate this site. I think I know where I'm going, see a new mark and get confused. The hate part comes from wanting to learn more about the pistol. The gun is made up from three different guns. Under the barrel are the numbers 2875 and the 75 is on most of the parts. However on the side plate is the number 42, then on the trigger guard the number 654. On the entire upper half of the gun is a crown with a n under it but on the right side of the Barrel Extension is a Waffen Amt proof. So was this a commerical pistol reworked to a military pistol? I'm also having issues figuring out what year it was made. I read that after 1910 they stamped the year it was made in to the pistol. This doesn't have anything like that. I also read the the crown over the n is the nitro proof used by the Weimar Republic, formed after the abdication of Kaiser Wilhelm, following the capitulation of Germany to end WW1 (treaty of Versailles). This happened in 1918. I know that the rework by Mauser and Simmons happened 1930-1933. So as you can tell I am completely lost. Also the holster is a nice black leather with a 1914 stamped on the back as well as C. Weiss, off to the right side the is S.H. 920. From what I've read the 920 stamp wasn't used til WWII. Any help you can give would be greatly appreciated. If you need anything like photos, numbers and what not feel free to ask.