It's a factory finish. In 83/84 I purchased new a Manurhan PPK/S made in France with the same finish. It was said to have a Duragaurd/nickel finish at that time. It by far is one of the most durable finishes I have seen. I have carried mine CCW over 20 years in some of the worst conditions, eg. cold, hot, sweat from my body and it just gets thrown in the top drawer until the next carry, I'm embarresed to say it's my only firearm I neglect and it looks like the day I purchased it. Works flawlessly every time I go to the range.
My ser #267xxx. That was imported through a florida based importer. My friend was a licensed dealer at the time, and we both purchased one from a small limited shipment that came in.
Yours is the first stamped Walther that I have seen.
I still have the original box and paperwork, I'll pull it out and see if it has any info on the finish.
kenny b