I must admit I hadn't been aware that these magazines were available as free downloads, but I had also posted these pins on the German SDA forum, and another member there kindly helped me out with information he actually found in that
Zivilschutz magazine:
The silver pin was for three years membership, the gold one for five and the wreathed one for ten. (Thus, the plain one is either a simple membership badge or perhaps presented after 1 or 2 years.)
The article is in the March 1980 issue. Thanks to your hint I found it online:
http://download.gsb.bund.de/BBK/Maga...hutz198003.pdf