[Thor]

John, I DO think that salt blued guns have a blued muzzle crown but rust blued guns DONT. If some of it is missing on salt blued Lugers it is from firing the weapon. I think the biggest tips for a reblued Luger is to look at the finish in the sunlight, especially rust blued (pre 1937 vintage) and look for browning under the bluing. Also look for wear on the high spots, the end of the front frame rails, the top of the sideplate, the sides of the muzzle end of the barrel, and the grip straps (this from handling) The gun will have been put in a holster so it will ahve wear there. Also if it says DWM or ERFURT on the toggle and the entire gun is BLACK or Dark black/ blue, including the safety lever, trigger, ejector, magazine release button and take down lever it NO LONGER has the factory finish (or at minimum the small parts have been blued over)
If it has plastic grips with BULLEYEs or targets on the sides and/or Xs on it, it is probably a Russian or East German Vopo (these will have small numbers in a starburst too uusually.
The best thing is to buy good books and go to lots of gun shows and view many many Lugers before you feel real comfortable in identifying original finishes. Looks also for RESTAMPED parts that have numbers that are jumbled up or too large or funny looking fonts. Good luck! It isn't easy to learn all this stuff, but it IS worth it. It takes time, research and patience!