My comments to the letters from Hal Davis and Jeffrey Borinsky --

Electrical supplies catalogs commonly list several different available
coil voltages for a magnet or solenoid.  There's no difference in the
performance (force & speed) of the high-voltage vs. the low-voltage
devices when they're controlled by equivalent circuitry.  The high-
voltage device may cost slightly more because it uses thinner wire,
which is more expensive for the same weight.

I believe that most of the Mills instruments used a "relay-style"
actuator which has a stationary core, versus the solenoid with a
moving core.  Could our Mills experts please confirm this?

As anyone with an old 1960s transistor-electronic organ knows, it's
difficult to keep low-voltage switch contacts clean when they're exposed
to the oils in the open air.  The Mills electric instruments use the hole
in the paper to allow a small wire "brush" to touch the brass tracker
bar, which closes the circuit to the magnet.  I believe that they
discovered that the standard low-voltage used in organs (12 to 15 VDC)
wouldn't always jump through any oil on the tracker bar, and so the next
logical system voltage was 110 VDC (still in widespread use in cities
then).  My old Allen electronic organ from the 1940s switches 125 VDC
very reliably with thin music-wire contacts.

Robbie Rhodes