Hereafter you can find a recent study performed by Paolo Petracco.
It is an important step for all the Luger collectors’ community.
Let's take a step backwards. When an assembler
had to finish the assembling of a P08(at this point already blued and
numbered and proofed) gun, he took any of the blued barrels "on the
desk”, he put the glue on the threads, he screwed the barrel into the
receiver, he put together the other parts thereby finishing the gun, he test
fired the gun with two high-pressure testing rounds, and, if everything was
right, he stamped the serial number under the barrel(the "halo").
If anything was wrong, he unscrews the barrel before the glue was hard and
screwed another one...