Ron and Ed,
This is interesting--the book I took my guess from was "The Luger Story" by John Walker. In it, he mentions 10 examples of unit markings on pre-1918 lugers, and one of those is the 139th Inf Regiment, with the marks as Ron suggests.
Thought I would also post one more picture of numbers on my 1917 Erfurt--the only place I can't make out the number is the firing pin. It still has a very golder hue to it, but all I can discern is what looks like two small "0"s, which may be the remains of the bottoms of the "5" and "8", or something else.
Appreciate your help on understanding a little more about this pistol.
