Luger id/info please

[NYCMedic]

A friend of mine has asked me to help him find out about this Luger and its gear. It was brought back by American soldier who (later in life) was a friend of his. Shortly before his passing the soldier gave him this Luger, in a holster, with a spare mag, and a second top half. For whatever reason, what appears to be the original top was separate and cosmolined and the newer top was installed.
I would love to hear about the markings (which I have attempted to photograph) and any info that you might have. Any ideas why the second top half? Are they different calibers? (I would assume that they would both be 9mm.) Is the holster military issue? I can not find any makers mark on it.

On a side note: I am a 1911 guy an know next to nothing about Lugers. I welcome all education on the matter. Also, I realize that there is a desperate need for a thorough cleaning and lube. None of my handguns are kept in such condition and I have already planned a cleaning session.


Thank you all for your help!