Minor Diapason is listed only by Irwin, who defines it thus: A Diapason of smaller than normal scale and less than average volume of sound, at 16', 8' or 4' on the manuals, and 16' or 8' on the pedals. It may sound in contrast to a louder Diapason or imply a Principal. The name Minor Principal is also used as a synonym for Minor Octave.
Only one example of Minor Principal is known, and none of Minor Diapason. Contributions welcome.