I may be very wrong, but when I worked for an organ company, I was told
that the Unda Maris (Moving waves)  pipes were tuned flat to create the
slow moving sound.  The Vox Celeste was tuned sharp.  (What wild things
would happen when the Celeste and Unda Maris were used together, along
with the regularly tuned pipes?)

Does anyone else have information?

Bruce Clark