I just called the Mercedes-Benz dealer here in Dusseldorf. Apperently the PZ number is the "personal identification number for the time of manufacture." Basically, as the clerk explained it to me, it's a number used to keep track of the engines being produced within a time frame. The other numbers, when scanned, help to determine the actual time frame. His best guess was that they change every month.

Look's like your's was an early one in December.