Maybe someone can shed a light on this interesting event in postal history.
I saw some months ago on eBay cover sent from Germany (BRD) to Soviet Union in 1969. As I remembered I have something like this myself I did take a closer look. It was described as "cover from postal war between Soviet Union and Western Germany" or along those lines. Seems I did not save photo. Here are scans of my cover:
Exactly the same label as on cover auctioned on eBay.
I found this article Der Spiegel 1969 but my german isn't much worth except it seems to speak about boycott or blockade. To the west of article you can see article in pdf format "Post/Briefmarken Blockade".
Perhaps a kind forum member from Germany could give a short summary?http://www.spiegel.de/spiegel/print/d-45741759.html
If anyone knows perhaps you can help with these questions. What blockade this was, why was it started and by whom and how long did it last? I have not seen any more of these covers.