Hi Jeb,
I assume you are talking about rust blued guns, mainly Imperial era up through early 1937 when chemical bluing was introduced. Marks and numbers that were applied before the rust blue finish should not show halos, those that were stamped after, should. It goes without saying that you should know the sequence of stamping and bluing of the gun or guns that interest you and this might entail considerable research, the best way to learn is by examining legitimate samples shown on Simpson's and Legacy Collectible's websites. However, if you are interested in unit marked Imperial Lugers it's a piece of cake, all unit marks were applied after the gun left the factory and all should show halos.
Norm
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