The timetable for the SXF to London flights in 1970 is here :
if70-08.jpg (1909×1338) (timetableimages.com)
The Interflug timetable actually included every flight into the GDR by foreign airlines. This was not uncommon in those times, not just in the eastern bloc but elsewhere as well - the Finnair timetable, so example, did the same for everything which went to Finland, whether officially in pool or not. I suspect the ABC was not allowed in the GDR.
Regarding flights not in the timetable, Interflug did of course do whatever the GDR government required for "special" flights, which could be all sorts. As elsewhere. At the time one would regularly find Pan Am 707s in Mildenhall. Which was never in the Pan Am timetable either ...