it stands for Stadt Duren

I asked the same question when I found out there were 2 types...the one with SD had a smaller mintage I was told.
Christian, from what I have read there was serious hyperinflation in places like Hungary and greece after WWII. In germany there was rampant inflation up until 1947 when it was concluded that there must be economic and monetary reform. Until 1947 germans were using occupation currency and, it seems, local emergency currency. Below is a blurb about post WWII Germany regarding its economy:
As agreed at Potsdam, an attempt was made to convert Germany into a pastoral and agricultural nation, which would only be allowed light industry. Huge amounts of factories were dismantled as reparations or simply destroyed.
Beginning immediately after the German surrender and continuing for the next two years the U.S. pursued a vigorous program to harvest all technological and scientific know-how as well as all patents in Germany. John Gimbel comes to the conclusion, in his book Science Technology and Reparations: Exploitation and Plunder in Postwar Germany, that the "intellectual reparations" taken by the U.S. and the UK amounted to close to $10 billion dollars--equivalent to around $100 billion dollars in 2006
With the repudiation of the U.S. occupation directive JCS 1067 in July 1947 the Western Allies were able to start planning for the introduction of a currency reform to halt the rampant inflation. This type of act to help the German economy had been prohibited by the directive and its execution also lead to the eventual division of Germany.
In 1948, the Deutsche Mark replaced the occupation currency as the currency of the Western occupation zones, leading to their eventual economic recovery.
In 1947 the Marshall plan, initially known as the "European Recovery Program" was initiated. In the year 1947-1952 some $13 billion of economic and technical assistance were allocated to Western Europe. Despite protests from many beneficiaries the Marshall plan, although in the less generous form of loans, was in 1949 extended to also include the newly formed West Germany. In the years 1949-1952 West Germany received loans which totaled $1.45 billion dollars.
The country subsequently began exporting local products, reduced unemployment, increased food production, reduced the black market, and slowly, but continuously, improved the country's standard of living.
By 1950 the UK and France were finally forced to stop the dismantling of German heavy industry.
So it might make sense that along with occupational money, local emergency money would have still been needed to fill gaps. Hyperinflation hit a lot of countries in Europe post WWII. Hungary is a notable extreme example.