Hi Mike,
I like this thread- thank you!
I can imagine it adds a lot to a particular gun to have a story behind it, and also to receive it from your grandfather. I myself have a 1917 DWM- very similar to yours, but with one very different stamp on the right side of the receiver. Interesting....
Here below is a link to that thread with photos....The stamp on the far left of your photo and on mine are distinctly different, while the other three stamps look identical.
http://forum.lugerforum.com/showthread.php?t=36379
Have you tried other types of ammo? Seems many Lugers are rather picky about the ammo they like... Would be interested in your experience... you mention only the Sig 124 gr.
Nice gun!!