[GreenKeys] M28 tape teletypewriter

[Jim Haynes]

Some guesses:  Prior to the Dial TWX cutover, for use on manual TWX
switchboards if they needed new ones after Model 14s were no longer being
made.

For use on telegraph test boards and service boards when they needed
the ability to operate at 100 wpm.

I'm sure our Bell System guys have better knowledge.  Do the parts have
Bell or non-Bell name plates?  e.g.  "28-A Keyboard" versus "LK-123"
Does the machine have a gear shift?

No gear shift and geared for 60 wpm would suggest TWX switchboard.
Gearshift would suggest telegraph test/service board.