I flew an Interflug charter roundtrip between LGW and Leipzig in 1981.
The food on the outbound was definitely ferried in from the DDR. I have a feeling they probably ferried in the fuel, as well.
Very pleasant flight aboard an 134. It had the glass nose! The toilet I seem to remember had a skylight.
As Interflug did not have rights to overfly west Germany, the flight plan took the 'plane across Denmark and then due south from Rostock.
In '82, I flew an 154 on a couple of domestic Aeroflot flights. IF was a class act in comparison.