Panda,just talking about the lokation of the "P" on Swiss Lugers - you are right, most of them are on the trigger guards, but have you seen one on the reciever already?
Here it is: that one is on a 06/29 red grip variation, as you can see at the serial number too.
Dwigt, of course the swelling can be seen on that "P" but here again, there is not to rekognize a real colour difference on the area around the new stamp! I agree with Panda, that it is not stamped heavy enough to cause a real difference in the colour.