Prussia’s first stamps date from 1850. They were engraved by
Eduard Eichens. Six years later, a new value was added to the set: the 4pf
yellow-green. Although the portrait of the previous values was used, Eichens
was no longer around to engrave the frame with the new value denomination, and
it was H G Schilling who engraved the frame for this particular stamp. Although
Schilling tried to replicate the existing frame as closely as possible, he used
the word pfenninge for the value, rather than the word pfennige which had been
used for the 6pf value of 1850. It is sometimes classed as an error, which is not
really true, because both forms were accepted at the time.
You will find H G Schilling's database HERE.