I just noticed a really good video posted on YouTube by Forgotten Weapons which demonstrates how to locate serial numbers on Lugers, which might be helpful to us new collectors.
If the link below doesn't work, just search for:
How to Check If A P08 Luger Has All Matching Serial Numbers
https://www.youtube.com/watch?v=fDTQ327u5gA
All credit goes to Simpson LTD & Forgotten Weapons
Zk