It is stereo out via jack with mono-stereo switch as it is anniversary electronics which are stereo. However, the configuration as 'series-upgradeable' only means that it has a hollow body and the correct routing for an upgrade to Series I or II electronics, so the 5 pin (and hum canceller) are totally dummy as stands and the bass can only be powered by battery (single 9 volt) not by power supply and lead. If the upgrade is ever undertaken, it will need a full set of 18 volt series electronics, pick-ups, hum canceller and 5-pin socket wired in at that time - an expensive operation unless you are transferring them out of a well-sourced series instrument. Hope that helps!