Hi Philip,
* After approximately the 2600n - 3200n range of '41 Luger production, virtually all Mauser Military frames bear a "P08" in the location you have mentioned.
* See the attached thread for transitions occurring in and about this S/N range.
http://www.gunboards.com/luger/topic.asp?TOPIC_ID=4017
* Some opine this P08 marking was added to "eliminate" confusion with the emerging P.38 pistols. I never did quite figure how anyone was going to assemble a P.38 slide to a Luger Frame and vice-versa; but, maybe with a Soviet T33 on a direct beeline, anything might have been attempted.
* Hope this helps.
Respectfully,
Bob