Jungendradio DT64 was a youth-oriented program which was produced by the Rundfunk der DDR. It was founded in 1964 and finally ceased operation as DT64 1986 in May 1993.
The medium wave frequency of 657 kHz was used exclusively by Berliner Rundfunk up to 1987. Jungendradio occupied the frequency after that. On FM before Antenne Bayern signed on to 102.7 MHz transmitting from Zugspitze, Jungendradio was audible on that channel near Munich. That would make sense because Sonneberg was just inside East Germany.
Rundfunk der DDR responded to my reception reports of 657 and 102.7 with the QSL cards you see below. I believe that the person handling the QSLs did not know or understand that Jungendradio was broadcast on 657 because they sent a card printed with Stimme der DDR on it. I will take it anyway.