To determine if the notch is there, you will need to lift the rear sight as high as possible without damaging it and then photographing the area beneath it where the upper receiver meets the barrel.
The "browned bare metal" look of the finish over the chamber is not consistent with the balance of the pistol and may indicate that a date was gound off and then reblued.
Your holster is intriguing and may have been re manufactured from a US Cavalry holster originally intended for the 7.5" Colt SAA by remodeling the closure flap to fit a Luger. Are there any stampings in the leather?
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