[PRD1]

Thanks for the insights (grin!)!
I could easily believe the pistol might have originally been an artillery: I had already been pretty sure it was a post WW1 rebuild. Another feature I didn't mention earlier is that there is a 'notch' and small flat in the front of the receiver ring, leaving the barrel shoulder standing above the receiver ring at that point: I can easily imagine that cut being a clearance/seat for the back of the artillery rear sight.
Those crazy Germans would do anything to make a buck after the Great War, and the American buying public didn't know any better...
I will indeed check and see whether I can get a replacement toggle with proper rear sight, which should help solve the problem.
As I don't have a reference on Lugers, is there any place I could view photos of the various toggle assemblies?
PRD1 - mhb - Mike