In his autobiography "Sand und Feuer", Drewes states that his combat planes carried no victory symbols (except for the first 4 victories). Kill marks appeared only on planes he used for non-combat flights (liaison etc.)
HTH, Richard
P.S.: Check out books on Drewes and other Luftwaffe pilots at
http://neunundzwanzigsechs.de/main.php
Highly recommended if you read German