[GreenKeys] Western Union Automatic Message Numbering Machine

[Jim Haynes]

That's quite a find.  Those were used in W.U. Plan 21-A automatic 
switching system which went into service in the late 1940s.  The system
is written up in a series of issues of Western Union Technical Review,
which is available online (Google for it)  Ordinarily they transmitted
a three-letter circuit identification followed by the number, e.g.
NSB041

For some reason they apparently didn't make any more of those, and then
went to a design using stepping switches.  Western Union Technical
Review 8:4 p. 135.  Seems like the stepping switch design would have
been a lot more costly, considering how much wiring it involves.  But
the writer says the reason was to get lower first cost and get greater
flexibility.