Two quick notes: The serial numbers rolled over to 1 at the start of a year, and went 1-9999, 1a-9999a, 1b to 9999b and so on. 1942 serial numbers ended at about 9999k, so 9999 times 12, or about 120,000 P.38's were made in that year by Walther. (By way of contrast, Remington-Rand made over a million M1911A1's from 1943 to 1945.)
The "359" was the number assigned to the head of the WaffenAmt (Weapons Office) inspection team at Walther; those numbers were assigned to a man, an officer specially trained in that duty, not to an office or a factory, and changed if or when the officer was replaced by another team commander. Obviously, one man did not stamp all the products turned out, though he was the responsible official; the actual marking was done by Walther employees under supervision of inspection team members. (359 remained at Walther through the war, presumably good duty, though he might have been elderly or disabled and unable to take other duties; the actual names of those officers was considered secret and AFAIK, no list has ever been found.)
Jim