I looked at the service manual over at Wild About Cars and it shows a three prong resistor?
The wiring schematic shows three wires to the resistor from the control too, brown-blue-green and the brown pigtails out and goes to the blower motor.
This does mount to the top of the heater case inside the car?
It looks like the A/C resistor type is mounted on the outside and is shaped differently so it isn't that.
The Canadian online parts catalog shows the resistor as part #9793206 for all 65-67, I did a search and some Corvette place has them and they're the three prong type.
Maybe they made a change during the model year or somebody coudn't find the right one and used that?
If that extra lug is common with one of the others that could be possible, does that single lug wire go to the blower motor?