Mojn
I think the red crayon is a capital "D", wrt below chart with different gothic handwriting styles. I have never seen a postal D endorsement before - could it be "Dienst" (Like a "KT"-endorsement in Danish language?), as the cover is from the German speaking part of SH ?
https://www.sa.dk/elearn/gotisk/alfabet.htm
Brgds
Ole