(Talking to myself again)
    OK.  I just got off the phone from calling Heath Educational Systems.
The only ETA-3400 manual they sell is 95 pages long, so that must be the
operations manual.  They want $35.00 plus $6.50 shipping for it, by the way.
$41.50 is pretty strong, in my opinion.  Fortunately, this is the one manual
I already have -- it's proper title is:
Heathkit Manual for the Memory and Input/Output Accessory for the ET-3400
Trainer, Model ETA-3400, part number 595-2271, copyright date 1979, 95
pages.
    If anyone has the manual (not sure of its exact title) describing the
modifications necessary to connect an ETA-3400 to an ET-3400, please email
me on or off-list.
    Paul Pennington
    Augusta, Georgia