Mike,
Nitpick away. My observation is that the amount of metal which is displaced upward is very small, compared to the area of halo. The halo varies a lot, and I have observed halo which, in area, is two or three times the area of the actual stamp--far beyond what might be expected to be accounted for by localized upwelling.
I reiterate my desire to see a metallurgical explanation of the phenomenon. It would also be interesting to see a comparison between compression and expansion of blued metal.
Alfred,
There is no single list. I think the best compendium, at least for German military Lugers, is found Jan Still's series of books.
In general, most original Luger markings--receiver marks, inspector stamps, serial numbers, worker's marks--are stamped before bluing. This is the same for military, contract, and commercial Lugers. Most barrel proofs, serial numbers, and caliber marks (but not inspector stamps) are stamped after the blue. Some are not, and the serial numbers on model 1900 Luger barrels do not exhibit halo even though stamped after blue. GERMANY export marks are stamped after blue.
Frankly, the before/after blue stamping on Lugers is quite variable. There are other kinds of stamps I have not mentioned, and they may be stamped before or after blue, depending on many circumstances. There have been many discussions on these Forums regarding marking halos, and whether the presence or absence of halo on specific markings on specific guns is proper or not.
--Dwight
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