tonearm ground wire

i'm trying to resuscitate an old hadcock arm on an equally old oracle. the tonearm wire had been altered so i replaced the 4 wires in the tonearm. where does the 5th wire for the din ground pin originate? as best i can tell from my son's rega, 4 wires enter the tonearm base but 5 emerge. to further complicate things, i'm using the phono stage of my arc sp9 feeding via the tape out to my arc ls5 (i know, what is such a schmoo doing with stuff he can't fix), do i need a second ground wire to preamp for the ls5 or is termination of the ground @ the sp9 satisfactory?
thanks,
jon