Melville Clark Apollo player with two action stacks

I recently ran across a piano that is likely a one-family-owned piano.
It is a Melville Clark Apollo, serial number 11505, and it features
a spring-powered roll motor. While these are not common (although
I have another spring-powered unit in the shop now), this one is even
more interesting in that it has a second pneumatic stack under the keys
for the mid-section of the piano.

It also has a very complex sliding "switch" on the back of the roll
box for selecting which stack is in use. So far I haven't found any
documentation other than reprints of ads for such an animal, but maybe
I've missed something.

The ads mention that it will emphasize the melody of the song, which
fits in with the second stack arrangement. There are separate hoses
going to the bass and treble ends of the main stack, but I failed to
determine if the stack is actually split.

David Dewey

 [ See "Melville Clark Solo Apollo Player Piano System," by Troy Taylor,
 [ at https://www.mmdigest.com/Archives/Digests/201002/2010.02.21.04.html 
 [ -- Robbie