Lance Reed posted in 200227 MMDigest:

"In original, untouched pianos that I have seen, these letters all
match on the three components.  Did the Standard Pneumatic Action
Company use these letters to designate some kind of a code as to
what year they were manufactured?  Or, what number they were in the
total output of production?  If anyone knows the answer, I would be
very interested to learn!"

I believe I know what these letters are about: it was the factory's way
of keeping the units together.  You see much the same thing on piano
cases where individual parts or assemblies have numbers stamped on them
that do not match the piano's serial number.  In the case of the cases
(I couldn't resist that! ;-) this was critical depending on the type of
wood and/or finish on the case parts as they progressed through the
factory, to insure they all went back to the same piano.

I, too, would be fascinated to see the machinery used to make those
thousands of identical intricate parts.  I know what I go through just
making replacement note pneumatic wood, creating various fixtures to
make consistent holes and other shapes in the wood.

David Dewey
Oroville, California