'73 would have dual unless changed. The dual isn't needed with the newer 4 pin ECU, but needed with the 5 pin. Although... I think the new 5 pin ECUs are wired like 4 pin internally, but I could be completely wrong on that... It would be a better system if it did though. If you look at how the BR works in "start" mode with an original 5 pin, you'll see that it's kind of Mickey Mouse and doesn't work as you think it does.
Post #12 explains it.
Question on Ballast Resistor
Single ballast with 4 terminals, put a new one on today but haven’t tried to start the car.
also just put a new 5 pin ECU on it 3 days ago, it started and ran ok, but today it decided to be a pain!