I have a very simple, un-researched answer.  Mason and Hamlin has
always been a very expensive piano and, like all manufacturers of the
time, they either made their own player or worked out an arrangement
with another manufacturer for a player action to be installed in their
own pianos.  In 1912, the self-player was the "home theater" of its
time and manufacturers almost had to be able to market a self-player
to stay in the running.

To my knowledge, Mason never made a player action of their own.
Some of the Welte Mignon "Red roll" pianos are Mason and Hamlin pianos
but American Piano Co. did not get Mason and Hamlin until late in the
game, about 1924.  Mason may very well have purchased player actions
from Cable Co. for their earlier upright players and this was Cable's
way of getting attention for its systems.

Steinway never made a player action of their own either.  Steinway used
Welte Mignon player systems as well as Aeolian Duo-Art.  Their 88-note
actions appear to be Aeolian from the few examples that I have seen.

Some food for thought on this subject!

Randy Hammond