I think Jim has struck the real answer to this question.
But, the stock lug isn't the only relic of the past which has stuck with the P-08.
I've noticed this odd little step machined into the left grip strap of all P-08-style Lugers, and never could quite figure why it is there (sorry about the quality of the scan).
Reading up on Simson Lugers this evening, I saw the picture of a Simson rework which actually retrofitted a grip safety onto a DWM P-08. I had a sudden epiphany, and checked a 1906 to make sure--all P-08s have the grip strap cut for a grip safety!
Another example of the specs (or the pattern) being this way from time immemorial, and the specifications were never chanaged?
--Dwight