I found a better explaination of how the war time P38's are serialized..maybe this makes better sense..

WWII coded Walther and Mauser P-38 pistols will also have a production year stamped on the slide. This is two digits. A Walther would be marked "ac 43" as example.

WWII P-38's were made in 10,000 gun "blocks". Each block was given a block letter code in addition to the serial number.
The first block made in January of the year would have the serial number and markings:
ac 43 0001.
The block would run up to 9999.
At that point the numbers would start over at 0001 but an "a" was added: 0001 a.
At 9999 a, the block would change to 0001 b, and so on.

This is important, since there will be MANY P-38 pistols with the SAME serial number.
What identifies individual guns, is the makers code, the year made, the serial number AND the block letter.