The surface is not flat, but roughed up. It needs to be in order for the enamel to adhere to it. This Roughed up, Gravel, or wavy pattern can be found on many translucent enameled badges as well. (not just bubbles - rice corns - stippling.
If you break the enamel off of good badges, (with opaque red) you will find an array of patterns, lines, score marks, and if the same die was used, then obviously the same die with rice corns or bubbles. The more opaque red badges you inspect, the more it becomes clear that a "change" happened, into the zinc period we also find makers using the same dies with "stippling - or bubbles " - just painted over, and towards the end of the zinc period you find that new dies were made, with no pattern and a different reverse made to accommodate lugs or other ways of affixing the attachment without solder. But they all have some form of pattern, even though you may think it looks blank, it is not. Enlarge a portion and you will see..
The pattern, is applied by hand, not by transfer when reducing the die from the master,(or working hub) the master (and hub) is blank. So if the die maker, or badge maker knew that he would need to only paint the badge, then he would save the work of adding the pattern(s) by hand and leave it blank. If they knew that opaque was being used (which at some point is was, used across Germany and the annexed terr. by most, or all makers at the same time) then there is no need to invest extra time in making a fancy pattern that nobody will see, and instead all that needs to be done is that the surface needs to be roughed up, or have "some" quick roughing up added so that the enamel will adhere.
I covered this in-depth in the
book, with many, many detailed images to best explain, so i cant go on here anymore about that. Just wait till summer next year! This "background" pattern Topic is covered extensively in the book, and i must admit, had me captured for many months. It`s not only mentioned, but gone into in detail like never before. The study of this small "element" alone, can contribute in a positive way in future evaluations, if it is correctly understood in relationship to manufacturing and period construction regarding equipment etc etc... BUT, you need really good images!!
This is the problem today with small images like the one you just posted, where to be honest, if you didn't know what you were on about, would lead you to reach 100 different opinions. When specifics are studied, in-depth, using magnified images, as well as a clear understanding of why you are seeing what you are seeing, it makes outing the fakes, Easy!