According to Dick's description of these pipes, they are in fact
vibraton pipes. Vibratons appeared in later Mortier organs in many
pipe ranks. I know of a late Mortier (Nr. 1081 "Electrosax") which
has these mirlitone membranes almost everywhere, including the stopped
bass pipes! I would call Dick's register a Vibraton 16' rank. Early
vibraton pipes had a round window, later on they used the oblong or
square ones with the dividers.
The original material for these membranes was a stuff called mica.
This was also used for windows in stoves, but I think it will be
hard to get now. Organ restorers here in the Netherlands now use
very thin Plexiglas, a kind of hard artificial glass.
In the next issue of "Het Pierement" of April 2003 there will be
an interesting article about this kind of pipe.
Success with the restoring process!
regards from Holland,
Hans van Oost, KDV