[GreenKeys] 28 Typer with Compressed Type and "ON-LINE STUNT SHIFT CONTROL"

[Jim Haynes]

>From the information in the W.U. spec this looks and sounds like the
thing commonly called "selective calling".  Such a machine has the
ability to suppress printing (and spacing and line feeds and carriage
return and maybe bell) while continuing to operate the selector and
the stunt box.  Used on multi-station lines so that stations print only
the messages intended for them.  In Model 15 days this required an
extra machine called a SOTUS to monitor the line and turn printers on
and off.  The manual version is an add-on to the online version, adds
a solenoid controlled by a toggle switch to override the print suppression
and allow the printer to print everything.

A big user of this feature was the FAA.  In Service A, the weather 
information service, they wanted to print everything from the line.  In
Service B, the flight plan service, they wanted to print only at stations
that were concerned with the flight plan being transmitted.  So selective
calling took care of the flight plan application, and then if they had
to switch the machine to a weather circuit the manual override allowed
that.

This was also used in switching systems for commercial users where there
was a need to have many stations on one line and wanted stations to copy
only messages addressed to them.  Aside from individual stations there
were group codes such that one group code would enable several printers.
A printer could have both its individual code and one or more group codes.

The same capability existed for Model 35.  However commercial customers,
especially Model 35 users, objected to the noise coming from the machine
when it was print suppressed but the stunt box was monitoring traffic on
the line.  It seemed to bother them that the printer sounded busy while
no message was being printed.  So that led to development of electronic
stunt boxes for selective calling.