As Ford puts it, the octane plug in some engines adds a feed-forward signal for spark timing and turbo boost. When the plug is in place in the underhood socket, it shorts the contacts in a circuit to the EEC for normal spark timing. If the engine is knocking with the fuel being used, you can change to a higher octane fuel, or you can remove the plug from the socket to retard the timing by the amount of 3 degrees, or reduce the maximum boost.
In a nut shell it is a safety feature the factory installed that if you are experiencing detonation the service provider can remove the plug to retard the timing 3 degrees. This was only employed from 93 - 98, some 98 vehicles and older still have the actual plug and wiring but it doesn't function. After 98 the function was moved to inside the EEC in the VIN block area where most tuner/programmers don't have access to electronically or can program. It isn’t a physical plug anymore, it’s in the programming. So once again it doesn't function.
The Mark 8 did get these and like I said above, you don't know what the plug is going to do when you remove it since there are several different EECs and calibrations. One of the possible reasons some Mark calibrations don’t use it is that these vehicle have knock sensors that detect any detonation and will retard the timing automatically.
The spout connector is the little plug on the wires going into the TFI module on the distributor or like some F series trucks, distributor run Tbirds and Mustangs on the inner fender or radiator support. You have to pull it out before setting the timing. Pulling it eliminates the timing advance function by disconnecting the ECU from the ignition system. The Mark VIIIs do not have distributors or TFI so they do not use a Spout connector but is physically there.
On the Mark VIIIs these plugs located either right behind the driver's side shock tower or under the plastic shock tower cover next to the DIS module.
