Think about the send-receive switch on a 15: up to send and down  to lock the keyboard by shorting it.

On my 15, two blanks in a row  will lock out the keyboard by mechanically moving the switch down.

I have read the reasoning is that if there are several 15's in a loop and one operator wants to send something important with being interrupted, he will hit two blanks and lock all keyboards. Then he will quickly unlock his and send his message. No one else can send until they manually lift their send-receive switch.

Mine also has FIGS-H motor stop. The motor is re-started by hitting the break switch briefly. Mine does not work. I need to study the book and stare at the machine to figure it out some day. It is not of vital importance to me.

Actually the KBD lockout is a nuisance. If I patch my reperf into the loop and want to run out blank tape with blanks, I have to hold the lockout mechanism to keep my KBD from locking. It is not enough to just hold the switch up. The bouncing shorting contact will transmit random characters.

Tom   N3CRK