Hello Will, welcome to the Luger Forum! You have a Mauser Military Luger, which is known from the byf German Ordnance Code. The serial number 6591o was completed about February of 1941. The inspection marks, called Waffenamt Marks, found on the right side of the receiver are both Eagle over 655. The 355 you mentioned is probably from a bad strike and is actually 655. The third mark is a firing proof mark and is an eagle over swastika.
The magazines shipped with the Luger would have been similar to the plastic bottom magazine you have except it would be marked 122, instead of fxo. Also the bottom would be aluminum and marked with the pistol serial number on the bottom. One would contain a "+" to indicate it was a spare magazine.
The holster is most probably a Police Style, but pictures will help confirm this.
Hope this helps a little.
