[GreenKeys] WU Tape Transmitter 7595-B

[Jim Haynes]

That's what is called a loop-gate transmitter.  It is documented somewhat
in Western Union Technical Review, but I don't have time right now to tell
you where.  It was used in, among other things, the Plan 55 switching 
system.  WUTR April 1958, October 1959, July 1960.

The purpose it serves is to solve the address absorption problem.  When
the system reads a message tape it reads the addressing characters to
see where to send the message.  But they want the addressing characters
to be included in the message, after they have been read.  The loop gate
transmitter with the reading head in one position reads the addressing
characters without letting them leave the transmitter gate, so the
tape goes up to form a loop.  Then the gate is shifted to the other side
so the characters in the loop can be read again, along with the rest
of the message.