You have a P-38 made by Walther in 1942.
During most of the war Walther used the code "ac" to conceal who actually was making weapons.
P-38 pistols were also made by Mauser who used "byf" and "SVW" and by Spreewerke who used "cyq".
Your gun was gun number 7552 in the "a" production block. That means it was probably made in January 1942.
P-38 pistols were made in 10,000 gun "blocks. The first block of the year had no block code. After the first block an "a" was added for the second bloc, then "b" from the next and so on until the end of the year.
Each block ran the same 4 digit serial numbers from 0001 to 9999, then started over with a block letter.
So, with three makers all using the same four digit serial numbers you can have literally dozens of P-38 pistols all with the same serial number.
The only way to positively ID them is by the serial number, block code if any, the manufactures code, and the year it was made.
So to positively identify your P-38 it would be "ac" 1942, 7552 "a" block.
Somewhere out there is a Mauser P-38 "byf" 1942, 7552 "a" block.
In each German arms plant was a Nazi government inspectors office that inspected the guns. These were known as "waffenampt" offices.
Each gun was inspected then stamped with a Nazi eagle and the number of the office. 359 was the office at Walther.
I don't know what the 355 code is for.
The actual serial number would be 7552 a.
The same serial number or at least the last digits should be stamped on the slide, the frame, the front of the barrel block, and on the locking block.
It's been reblued, because you can see pitting on the right side of the slide that's been blued over.