[GreenKeys] So what else can I do with this?

[Jim Haynes]

Some Model 15 bases have a motor control relay mounted on the base, to
the left of the fuse.  And some of them don't have the relay but have
loops in the wiring for connection to the relay should it be added.  And
usually those that have the relay don't have it connected, but the
loops of wire are there for the purpose.

I'll confess I have never understood how that relay was used, have never
seen one where it was wired into the circuit, and don't know what it was
connected to externally.

A different option was a motor control contact operated by upper case
H.  This was mounted on the low front of the typing unit where it was
operated by the function mechanism.  Then there was some linkage to the
selector magnet so that the magnet going spacing would turn the motor
back on.  I don't think I have ever seen one of these in working order,
but have seen the parts.

The send-receive-break mechanism is not involved in motor control.  It
is tripped to the receive position on detection of a blank by the
function mechanism, the result of the getting a break on the circuit.
In the receive position it shorts out the keyboard so you can't send.
In break it opens the keyboard so you can send a break.