Purpose of "Diapason Bass" vs Diapason, or Melodia, and associated "Diapason Treble"

[John Edelmann]

On the treble side of my organ, I have a celeste, and a melodia stop. But to the right of the melodia is a "Diapason Treble" that really only appears to open the mute for that stop "wider". Similarly, the Diapason on the bass side has a Diapason "BASS" that appears to do the same thing. It's unclear to me what it is that the Bass or Treble version of the same stop really does... though the sound is clearly louder/richer. I'm curious as to what is mechanically actually going on...

[SubBase]

Manufacturers went to great lengths attempting to get more than one quality of sound from a single set of reeds. An extra stop knob and the associated links were cheaper by far than an additional set of reeds and all that went with them. Some of these efforts are more worthwhile than others, but the half-open mute, known as a "soft stop" was universally employed. The subtle flatting of pitch that results from opening the mute a bit less has another benefit, that of making a chorale effect when played against the other set of reeds of the same pitch that are fully open. This benefit accrues only when the reeds have been cleaned and tuned of course.
As far as your specific stop names, these vary by manufacturer.

[John Edelmann]

Thanks Sub ... that's the insight I was looking for... makes total sense!