[GreenKeys] Folks, what exactly is going in and out of this 'thing' under my TTY's stand

[Jim Haynes]

That thing is simply an electrical service unit, and collects together
the power connections and the signal connections.  It does not contain
a loop power supply, and looks like it does not provide for one.  Those
things come in various configurations.

At the left end are jacks for the red (printer) and black (keyboard)
cords from the machine.  They are wired in some manner to those terminal
blocks underneath the unit.  Then in the middle is what looks like the
four-wire outlet and cord for power to the machine.  The four wires
are safety ground, neutral, hot lead, and hot lead through the power
switch on the machine.  The outlet on the right may be just a convenience
outlet for your trouble light and soldering iron, or it might be switched
by the power switch on the machine.

Some variations on the unit include an AC outlet for the power supply,
a DC connector for the power supply, and an outlet for a DC cord from
the machine - bringing in DC to use with a line relay or a line/test key.
But yours doesn't seem to have those.