If I'm not mistaken, the photos seem to show that the stock lug has been removed on this pistol, which indeed appears to be a matching 1917 DWM (you got your 1 and 4 mixed, serial number ends with ..41). It is not uncommon to see removed stock lugs on pistols that made it to the US, had to do with some misinterpretation of local legislation, I believe. Although there have been reports of lugers that had their stock lugs removed in Germany as well. The serial number should be present on the front of the frame, most likely with a small script letter underneath it, which is part of the serial number.
I would suggest unloading the magazine. Keeping it loaded for extended periods of time is not very good for the spring itself.
If the ammo is of pre-1945 vintage, it would be interesting to know the text on the headstamp (bottom) of the rounds.
Furthermore, I would advise not to store the pistol in the holster anymore. Holsters are moisture traps and will harm the finish, as is visible on your gun. The gun needs some thorough oiling
