Correct. That was the jack for a Discrete Quad FM adapter.
As far as a Quad FM standard, one was approved that allowed for multiple surround FM systems - including discrete. But the approval came so late that Quad itself was dead by then. So the adapters never came to market.
>>I'm not a quad expert, but I believe that the "quadradial" output was marantz's equivalent to the MPX OUT found on most quad receivers (well, my Toshiba has a MPX OUT). This outputted a non-demodulated FM signal (ie - the FM mono signal) and was meant to allow for a 4-channel FM demodulator to be used once a standard was reached (but this never happened). This would allow users to use their existing FM tuner and they would only have to buy the demodulator and hook it into a 4 channel input.