To me it looks like the original surface preserved by the original paint having been there and which got removed during the cleaning process. The not covered part of the case and primer just corroded over the time.
Try a close look with a microscope.
Any other thoughts?[/quote]
Privet, EOD! In Ukrainian we even can say (in that case - write) - Pruvit.
Yes, it looks like removed varnish. But only one person who clean this case was I am. I clean only some spots of white patina from the base of the case (I clean it with small knife) - to see the headstamp. And I didn’t touch the primer and its border. I ask about this strange case, because I already met the same samples some times ago, also with marking DM-S-14-9. And this samples also was without black varnish. At that time I thought that varnish was missed during cleaning, etc. And now I met the same sample from another source and also with strange transparent line around the primer. I am absolutely sure that nobody clean this cases, beside me.
Another interesting moment in this headstamp - is very deap marking DM, usually it very thin.
Some day soon I will try to get another cases from with this marking and place a picture in this topic.