This is a fun subject which I've followed with great interest and some
amusement. From experience, the idea of keeping a Band Organ in tune
long enough to appreciate the difference between even tempered or
unequal tempered scales is amusing in itself. The modern (equal
tempered) scale is based on the idea that each octave is doubled in
frequency and that the intervals between octaves are made up of
intervals (notes) each increased by the 12th root of 2. This system for
tuning organs began in 1683 and by 1854 all organs were tuned to this
even tempered scale. Untempered intervals are still used only in
harmonic mixtures where multiple harmonically related pipes make up
individual notes. This creates acoustical equivalent of a fourier
synthesis of individual pipe waveforms into a complex waveform that
sound different from the individual pipes. Gavioli adapted the equal
tempered scale in 1887. Bruder adapted the equal tempered scale in
1899. Much (or possibly most) of the band organ builders in the US (the
North Tonawanda groups) learned from Bruder. I have no doubt that one
can tune a 150 scale organ by some of the unequal tempered scales but
the difference from equal tempered tuning will hardly be noticed after
about 30 minutes of play, and certainly not after moving the organ to a
rally site. I suspect that any tuning 150 scale organs to an unequal
tempered scale was practiced out of convenience. All one needed was a
starting note reference and the rest was done by ear. I really doubt
that this was done because it somehow sounded better. Most of the
tuners that I know now use electronic (equal tempered) aids as a
reference. Just thought I would add my 2 cents worth ...