They are talking about the protruding bit that goes into a wooden stock (or a metal IDEAL but not many of those around). It is at the bottom rear of the handle/bottom of the backstrap(near the mag well).
It does indeed look like it has been ground off, but that might just be a trick of the angled pics.
Do you have a pic of the bottom rear of the handle, please?
In the most recent pics you uploaded, there is heavy wear on the backstrap and what might be mistaken for the start of a grind track ..., if one had a vivid imagination
Not all Lugers have the stock lug. Many of the old production ones are missing it, like the ones that were made for commercial use and/or the Swiss, Bulgarians and a host of others for foreign countries. Certainly a S/42 Luger like yours, if number matching, must have the stock lug as all the military P 08s have...
Best regards, Thanos.