[GreenKeys] Brecom system

[Bill Strangfeld]

Someone was just telling me about the "Brecom"
system developed in or around WW2.  It involved
hidden FSK keying on clear-channel am broadcast
stations in Cincinnati (WLW) , St. Louis, Des
Moines, Salt Lake City, San Francisco, Los
Angeles, Dallas, New Orleans, Atlanta, New York,
and Pittsburgh.

They keyed the carrier of these stations by two
cycles at 30wpm.  It was unnoticeable by regular
listeners.  This chain of stations was a backup if
national telephone communications failed.  It
apparently wasn't needed for the design purpose,
but they kept chatter on the system to maintain
readiness.  Apparently used model 15 machines, but
my informant wasn't certain.

Anyone have more information on this?