Some interesting theories, but a much simpler explanation. The Luger (Parabellum) pistol was adopted in 1908 (hence, Pistole 1908 or P.08) but uniform marking standards were still being worked up and the earliest date marked was 1910. That pistol is one of about 20,000 made in that period. The three digit serial is legitimate, it is the 592nd Parabellum pistol made for the German military, and IMHO, a very valuable piece, especially in that condition.
JIm
JIm