I used to own an LP of the "Dichterliebe" recording. That recording, made in 1964 or 1965, was in stereo, and is a wonderful performance. The Kerner-Lieder recordings were made in 1957 or 1958, and I presumed that they were stereo as well. (Stereo recordings were available as early as 1954.)
Well, it turns out that the Kerner-Lieder recordings were mono after all. I was extremely disappointed, and almost returned the disc because of that. However, the performances were so good that I decided to keep the CD; the recording of "Dichterliebe" is not available on any other single disc.
If you must have Fischer-Dieskau's recording of the Kerner-Lieder in stereo, they are included in his 6-CD album made in the 1970s with Christoph Eschenbach as accompanist. Those two are wonderful performances.