Some of this is in the FAQ - you can find that in the New Collector area
http://forum.lugerforum.com/forumdisplay.php?f=133
P38 / P08 - same idea behind the codes, it was to disguise the factories. Goes back to the beginning of the war. Not sure what books you have, perhaps they just assume you know? But it was a thought out, but made up code. Same with the acceptance markings (not proofs) on the right, eagle 655, eagle 63, earlier, crown N, crown B, etc.
The letters are in detail in the FAQ, but they represent each 10,000 made - so first 10k is NO letter *suffix) then next would be 1a, 2a, 3a, up to 9999a, then 1b, 2b, etc