[RolandF]

You are right, of course, Anton. But those permanent StKzs were applied mostly after a plane returned to secondary service after front service. Mostly after repair and being handed over to semi-operational units, flying schools etc. Oftenly the letters were enhanced by adding a white trim.
The disapperaing of washable StKs can be seen on late Fw 190D V-Muster which were taken out of production. After some months the StKz was nearly invisible due to weathering. A good example is WNr 170003 Fw 190V53 DU+JC used for the D-series development.

Regards
RolandF