The Foster & Co. keylock is a very simple device.  It is constructed of
a small 1/2" x 3" board that is as long as the keybed.  This board is
hinged in the back by five small common hinges.  If you look carefully,
there will be evidence of these.

There was small metal bellcrank fastened to the board on the left side.
When the knob was pulled, the board simply stood up edgewise, taking up
the space between the bottom of the keybed and the keys.

Most of the time the board was felted with thin felt to prevent the
keys from knocking, although in a few examples, especially around 1913,
the bottom of the keys were felted instead of the lock rail.

Andy Taylor