Rothhammer, there are some misconceptions in the above table of German proofmarks used 1893 -1939, as published by Stoeger in 1939:
The German Imperial eagle was the mark for primary proof of the rough bored barrels alone. Rarely on post-WW1 guns. From the start primary proof was not mandatory. At the times of hammer welded damascus barrels it was for the sake of the gunmaker, to keep him from investing work into faulty barrel blanks.
The crown/B = Beschuss = proof load fired mark is not restricted to "foreign arms". It is the mark for the final proof.
The crown/U = Untersuchung = View mark stand for the inspection after final proof firing.
Crown/G = Geschoss is for guns using a solid projectile, mostly rifles, but some smoothbores too.
Crown/N stands for Nitro = smokeless proof, of course.
So a German proofed rifle barrel usually shows four marks, crown/B, crown/U, crown/G and crown/N, BUGN in short.
Exceptions: The Oberndorf proofhouse at the Mauser factory before 1918 marked commercial sporters on 98 actions BU only. All 98 action arms then were rifles and used smokeless cartridges. Why mark the obvious? After the end of WW1 Geha and Remo shotguns on 98 actions appeared, so Oberndorf marked the rifles BUG. In the 1920s Mauser started to build Wehrmannsgewehre on 98 actions for the originally blackpowder 8.15x46R target cartridge. From then on Oberndorf used the full BUGN set.
From 1893 to about 1923 an alternate set of proofmarks was used on some rifles. The marks were a big crown with the crown/N mark besides. These marks indicated proof using the special smokeless "4000 atm proof powder".