In response to John Tuttle's question of the key lock device in
players,  I think that Art Reblitz is correct; the lock was there to
make the piano action respond to some of the rolls that have a lot
of tremolo (repeating notes) and where some people prefer to play the
rolls  really fast.  The key locks purpose is to take the weight of the
key out of the equation, so the rest of the piano action will then
operate faster.

The main limitation in a player piano for note repetition is the piano
action.  If you play a properly restored player stack on the bench and
put a test roll on it and play the stack with an external source of
suction, the pneumatics will just buzz up and down very fast.

On the old upright players from the 1920s, and non-players too, the
touch weight is quite heavy.  I've measured between 60 to 70 grams of
touch weight in the tenor section of the keyboard.  On a properly
regulated grand piano, the touch weight is around 55 to 60 grams in the
tenor section of the keyboard.  They purposely made the touch weight
heavy, especially on players, to allow the key to return faster to its
rest position.  They did this by adding lead weights to the back end of
the key.  This would allow player pianos to repeat faster.

I installed a PianoDisc in an old upright (1907) which had about 65 to
70 grams of touch weight, and it repeated notes fairly good.  Several
people complained that the piano was hard to play.  So, I did an
experiment and decreased the touch weight to 55 grams.  Then the piano
action didn't repeat well at all.  The notes were stumbling over
themselves.  Piano players loved it when playing slowly.  But, when
playing the PianoDisc with fast pieces, you could see the piano was
missing notes.

I leave players alone with their heavy touch weight and use the key
lock if you really want it to repeat fast.

Bing Gibbs - Player Piano Forte
Volcano, Calif.