Due to material shortage in World War I and the aftermath of the war till 1921 official mints couldn't cope with the task of striking huge amounts of coins. So many coin mints often run by municipalities and private contractors struck local issues on inexpensive waste material of metal processing companies.
They were accepted by the "Reichsfinanzministerium" of the government and became legal tender if two criteria were met: the municipality striking its own coins had to pay the full amount of money to be locally distributed equally in Reichsmark to the "Reichsbankfiliale" and had to give to the "Reichswirtschafts-Ministerium" in Leipzig a coin of each different type they planned to strike as example. Only these officially listed coins were accepted as legal tender throughout the German Reich and could be traded in any of the "Reichsbank" bank offices for official money.
As local mints in wartime had to deal with inferior material in both coin planchets and striking machinery, lots of dies did break and had to be exchanged for new ones. The result is a large amount of varieties on "Notgeld" and "Kriegsgeld" emergency coinage.
Collectors started collecting emergency money even in the years they were used. Since many municipalities and even a lot of local public transportation and factories made their own money (coins and banknotes) it was a vast field to collect. However, in the aftermath of WW2 many collections did vanish. That is because the eastern parts of Germany were overrun by soviet forces and the German population fled or was expelled. So the genuine mintage figures given don't reflect an actual amount of coinage left to collect.
Even after WWI in the early 20s many emergency coins got restriked to satisfy the collector's needs. Those coins were copied with original dies of "Chr. Lauer" mint of Nuremberg which was responsible for a lot of issues from different municipalities.
However, issues on expensive material like gold and silver might not be counterfeits or restrikes but are rather to be considered very rare. Gold and silver strikes of emergency coinage where often made on request for the municipalities' mayors. Later on different mints offered these special coin strikes for collector's, too.
This is an edited and extended translation of information that is given in the "Funck Notmünzen"-catalog for Emergency Coinage of Germany