It looks like it might be a companion piece to another hunk 'o furniture and electronics that looked like that but contained another mono amp AND front end, possibly consisting of a tuner, phono and preamp.
They could buy the other unit and it all they wanted was mono, they were done. If they wanted to upgrade to stereo they would buy what you have.
Now, if you had another one like that, they would make one hell of a computer speaker setup.
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“He who dies with the most toys is nonetheless dead”
Philips AF-887 TT, Marantz CC 4300 CD changer, Teac R-400X cassette
Marantz 2270
JBL L-26
AV Adjunct: Toshiba 27" TV and DVD/VHS combo
Other toys:
http://cgi.audioasylum.com/systems/1606.html