Eric -
To answer your question a bit more directly, the stock lug was added somewhere between s/n 4918a and 5609a. From time to time I look for more guns to help narrow down the transition range for both this and the hold open transitions. Any assistance by forum members would be greatly appreciated.
I think the 1913 Erfurt is a fascinating study due to the transitions going on at this time in Luger configurations.
http://luger.gunboards.com/showthrea...en-transitions
Mark