There's another one of these babies on ebay for sale with clear photo front and rear.
Vintage Qm-800 Quadraphonic Power Amplifier Item # 1943407450
There's definately no tape loop returns here, just 2 sets of 4 RCA jacks labelled "INPUT" which can be selected from the rear panel. Only outputs are high-level speaker outputs via those oddball pioneer jacks.
So it genuinely is a simple 4 channel power amplfier!
Actually, at a fairly reasonable price too - this one's in nice nick, boxed, too.