[mrerick]

The Crown/U proof is from the Oberndorf proof house. The E/N proof is for a commercial nitro proof.

I have one of these built on a 1938 receiver, and midway through the "U" block, in .30 Luger caliber.

They are quite rare since Mauser was fulfilling huge military contracts when these receivers were made.

I'm not sure that they were all made in the year of their receiver, since they may well have been made from parts held back from military contracts for one reason or another. The finish variation you see on this one is evidence of that. In particular the nickle alloy content and salt bluing chemical balance that lead to the more plum color of the toggle train.

Also, by 1939, for example, a throwback to Imperial and Weimer eras, should the Crown over letter proofs still be be in use?

The "U" "V" and "W" blocks were reserved for a number of commercial contracts by Mauser.

I can't explain the "Made in Germany" barrel stamp except that these may have been made for export to the USA. The US import laws we deal with today are different from the earlier ones that only required country of origin markings. Mauser made Lugers for other countries like Sweden were not stamped in this way.

I know that Don and Joop document these commercial guns pretty thoroughly in their book on the Mauser Parabellum. The one I have in the "U" block is very late in the series, and probably was made for Swedish export.