My favorite overprint is my avatar.

It is a "Bezirkshandstempel" from Soviet Occupied Germany. With an unexpected revaluation of the currency on June 21, 1948, all the current stamps were demonetized.
There was not enough time to design and print a new series of postage stamps. The post offices were instructed to apply their local hand stamp, in German, a "Bezirkshandstempel" on stamps sold from their existing stock to indicate that the new rate was paid.
With 20 denominations of stamps, different colors of hand stamp ink and a unique hand stamp from every post office in Soviet Occupied Germany, it is estimated that there are over 30,000 possible different stamps in this series.
These stamps were only valid until a new series of overprints was issued on July 3, 1948. Because of the brief timeframe, enterprising postal workers hand stamped their remaining stock of stamps for sale to collectors.
My avatar is from my set of stamps from Sommerfeld, a town in the Potsdam (36) district. The town was probably a wide spot in the road in 1948 so although the stamps are genuine, it is most likely that my set was made for collectors.
I am partial to this set not only because I collect Soviet Occupied Germany but also the town, Sommerfeld, is basically my last name, Sommerfeldt!

