I'll echo your comments about Dwight - read this thread about how I feel about folks that undertake their own research, and Mr. Gruber's efforts in this area are unparalleled.
OK - remove the receiver from the frame, and on the underside you may see the witness mark - which is a stamped "groove" that aligns the barrel with the receiver. Dwight's research can be read here:
http://www.lugerforum.com/WitnessMarkConclusions.htm
which John S. just updated on the site for all of us.
My Best to you Doc - always,
- John