Triggerhappy,
A post at this link
http://forum.lugerforum.com/showthre...t=firing+proof explains the marks on the right receiver.
Your new photos, although still too dark, clearly show that this Luger has no stock lug. Jan Still (Imperial Lugers) describes 1913 as a "transition year"; the stock lug shows up late in 1913 production, and a very few are reported manufactured without a holdopen. Estimated production of 1913 DWM was 35,000 pistols, if your example has no letter suffix under the frame number it is an early example and the added holdopen makes sense.
SA stamped inside a box on the left receiver is the Finnish military property
mark. If your Luger does not have this mark it is most likely that it was rebarrelled in Finland by a private owner.
Finland did not adopt the Luger as its official military sidearm until 1923; most Finnish Military Lugers came from Alphabet Commercial production, and are found proofed on the left receiver with an upright crown/N German commercial proof, along with the SA Finnish property mark. Finland maintained their Lugers aggressively. "Finnish reworks" are characterized by some combination of: mismatched parts; modified (square notch) rear sights; Tikka barrels; and a very matte, almost "Parkerized"-looking finish.
--Dwight