The red cheat line was certainly part of the Inter Ocean colour scheme and they were the only ones operating the DHC4, apart from the RENAMO rebels who had a single example which I doubt went anywhere near government controlled airports. Given the age and general condition of the IOA fleet, I doubt they would have been worth much after the war and with the amount of money the owner made he could easily have afforded to walk away from a few derelict airframes.
Nampula was also an aircraft graveyard, mainly military though with plenty of early generation MIGs lying around.