Hi Farkas,
thanks for the scan of the reverse of he card. It was indeed mailed, but in an envelope.
This is a rough translation of what was written:
Save this card for me and keep it, it shows the grave of our NCO-medic (Sanitäts-Unteroffizier), a soul of a man who died in hospital of his illness on the 30th of March this year. Soldier Rub seems not to be listed either. There are two Emil Rub listed, but not with corresponding dates. He must have been from the XIII. AK, Landsturm Bataillon Gießen.
GreyC