My (wild) guess is that the terms are derived from organ valves, where
pneumatics had been used for quite some time.  And there, valves had
a typical orientation in that the valves opening the ducts to a pipe
were lying below the corresponding pipe, and often also drawn like that
in diagrams.

I believe that many diagrams for player pianos followed the "customs"
of organ pneumatics drawings, and from there, upper and lower were
taken as "what the designer thought was most corresponding to the organ
schematic/diagram case".

I am reminded of the discussions of software people of whether a
"stack" -- not the piano stack but the LIFO (Last In, First Out)
push/pop stack -- grows "upwards" or "downwards", and, correspondingly,
what the "topmost" element is.  There is no real agreement here, only
conventions which hold more or less.

Or what about the convention that plus or positive voltage lines are
typically drawn on top of an electric diagram, and ground is minus and
is drawn on the bottom.  This lead to an age of confusion in the 1960s,
when only PNP transistors were available which needed a negative supply
voltage in typical circuits.  Would now minus be on top, or would
(positive) ground be on top?  (Fortunately, NPN transistors were
invented soon after ...!)

Okay -- just guesswork ...

Harald Mueller