Curiosity Corner #213: One to look out for.

SGE1823
expertised on back by Herr Helmut Wiegelt's signature,
The signature may be genuine, but the stamps are forgeries.

His signature was applied in the summer of 1977
when insufficient information was available, and the stamps
were considered to be errors of colour.

Source: Otto Hornung, "Here and There" October 1979

(The interesting point is, one can change the colour of the
genuine stamp, without affecting the freshness)