Excellent question, Don.
That is a common misperception for 1972. I don't know about the 455HO GTOs, but the 1972 455HO Firebird Formulas got the semi-functional, unitized ignition as a mandatory option. On the Trans Ams it was an available option. Thankfully, the original purchaser of my car didn't order that option (though it took me three years to locate an original 1112126 distributor).
For the bystanders out there, the unitized ignition was a prehistoric ancestor to the HEI distributor, used in 1972 and 1973. The coil packs in these units would heat up and then stop working, usually in the middle of an intersection or on a highway with a convoy of tractor trailers behind you. Also, the parts were insanely expensive and only available for a few years. It used an octopus-like set of plug wires with all eight wires permanently attached to a bracket that would install on the top of the equally impossible to locate distributor cap. Here's what the wires look like:
If you dont have the PHS or build sheet handy, the way to tell is with the engine code on the block (assuming you have the original block).
Here are the block codes:
WD = 455 HO 4-speed with 1112126 points ignition distributor
WM = 455 HO 4-speed with 1112133 unitized ignition distributor
YB = 455 HO automatic with 1112126 points ignition distributor
YE = 455 HO automatic with 1112133 unitized ignition distributor
My original engine has the WD block code.
