"Mors-mors" is only the second part of a saying when Hamburg citizens used to welcome (and identify) each other in a funny way when meeting abroad.
Itīs a question-answer game - the question being "Hummel-Hummel?" and the answer "mors-mors", when, indeed, a Hamburg citizen was involved - at least the answering one. Using the inscription "mors-mors" you show your Hanseatic background imo.
The saying comes from the beginning of the 18th century and describes a top-hatted water-carrier called Hans Hummel, carrying two drinking-water buckets hanging from a yoke across his shoulders.
This Hamburg original is depicted as a black silhouette confirming the Hamburgian background of the pilot or maybe the squadron - and yes, the direct translation is "kiss my bacxide" with the meaning "Get stuffed!"
Remember the He 111B Pedro 1 "Hummel-Hummel" 25-9 flown in the Legion Condor.
Regards
Roland