Around 1970, the lightweight Sennheiser headphones were
out, and cassettes had been around for three or four years.
Players (which were all recorders, as well) were still
about 2x6x11", and the D cells took up a fair amount of
that space. But Sony put out its first stereo portable
cassette recorder in that format, about that year, maybe
'71. It looked slightly bigger and fancier than the usual
mono ones. And for stereo, it included a headphone jack,
as well as the usual mono speaker. Looked kinda like this,
but somewhat slicker, as I think that is a later model, and
I think "Superscope" was Sony's budget brand, while this one
was top of the line, being the newest thing:
http://www.derekspratt.com/Images/Audio/SuperScope%20CS200S%20Cassette%20Deck.jpgSo, you could put your cassette deck into a backpack,
with the headphones plugged in, and walk around listening
to the Moody Blues new album, on a sunny afternoon in the
park, almost a decade before the "Walkman" came out. It
was a completely novel experience at the time. A bunch of
us kids put this together, one had the phones, another the
deck, and we took turns using the device, as it was in
high demand, being an utterly novel experience for the
time.
So it was kinda cool, when the walkmen came out, to be
able to say, Hah, we've been doing that fer years.