[Tony Kambic]

Stammkennzeichen (unique four-letter factory code, assigned to the aircraft for life) . All alpha characters, no numbers.

Verbandskennzeichen (four characters unit code, assigned by unit). I believe this always had at least one number.

WNrs. generally did not change, since it was stamped on a plate attached to the airframe. There has been some discussion that airframes were modified/rebuilt, as in the case of changing a radial engine Fw190 to a long nose one, and when the airframe was modified the WNr was changed.

You can find plenty of discussion on the topic of Stammkennzeichen on forums. I also find Truppenkennzeichen used for Verbandskennzeichen.

Tony