I don't know dick about mustang wiring, but on my cars, they all run 4 wire 02's.
2 wires are for the sensor itself (output& gnd) and the other 2 are for the sensor heater to get the sensor up to operating temp faster.
I can theoretically see a vin split where ford started using a 4 wire heated sensor vs not perhaps ?
In looking at the picture though, Cordell would be spot in, given it has it's own separate wire (that would appear to be something that would plug into a typical 1 wire oil sender) it's likely just taped together.