The "X"

I have a 1939 Luger with an X stamped or engraved on the bottom of the frame in front of the trigger guard , below a 3 digit #.[I assume is the serial #] My father brought this gun home from WWII. He came home in 1944. I have been trying to get more info about this gun, nothing seems to come together. It has matching part #'s. [Mauser] logo on top, 1939 stamped on the receiver on top, and a German Eagle with an X inside of a circle with a letter L in front of the logo. this is on the right side. On the left side of the receiver in front of the trigger above the serial # is a logo horizontally a crown with a U under it. Any info would be appreciated. thanks Ed.