What Bob Fitterman saw on the Hooghuys at Utrecht is a 'Mandolin'
register, which was fitted to his later dance organs.

Although termed a Mandolin, the strings were struck by beech wood
hammers (as in a dulcimer) and with the very rapid action that Louis
Hooghuys was noted for, produced a fast enough tremolo to give a
Mandolin sound.  Some organs were also fitted with this in the
counter-melody, where it was known as a Guitar register

Due to the fact that it as not very loud, and tended not to stay in
pitch with the rest of the organ, they soon fell out of favour and
were either disconnected or removed completely.

The only example of a functional Mandolin register can be found on
no.  595 (built 1910 and the Mandolin removed in 1914) which is owned
by Ted Bowman in England.  This has been rebuilt from parts of Mandolin
and Guitar frames and hammer chests found the loft of a house owned by
the late Roomain Charles Hooghuys in Zeebrugge following storm damage
in 1990.  Unfortunately the Utrecht organ does not have the hammer
chest fitted and the frame and soundboard moved to the front of the
proscenium.

Jeremy Brice
Watford, Hertfordshire. U.K.