The MKIII was the first edition they released of the JCM 900 model. It was manufactured between 1990 and 1992, before releasing the Dual Reverb and the SL-X. In practice, it is assumed that it started from the circuit of a JCM 800 with one more diode gain stage (come on, like a built-in pedal to reach more power).
It has two masters that are selected by pedal and the "normal" gain called preamp that goes from 0-10. Then it has another pot called gain sensitivity that goes from 10-20, which is where those diodes come in. Full tube design and sound with ECC83 and 5881. Within the JCM 900 it is considered the holy grail, for being radically different from the Dual Reverb, and being one of Marshall's hidden gems, as has been discussed and claimed in specialized forums and social networks in recent years.
