My wild guess... You have more than a single wire routed there by a previous owner. 67 and earlier automatic trans builds had the reverse lamp switch inside the cab on the shifter. A single brown wire for neutral safety routed from starter relay in engine bay down to that single function switch screwed into the trans.
A later model trans has a dual function switch screwed into it. The single neutral safety switch wire and 2 reverse lamp wires all go there. So.... Someone installed a later model trans and routed the 2 additional wires from inside to outside and under. When the factory did this change they routed these 2 wires through 2 of the previously unused ports of the bulk head connector and taped in with the neutral safety wire. You discovered someone alternate method of routing.
So if I'm right, trace these wires back under the dash and find them attached to a purple and a purple w/white.
