The Voyager mission status bulletin 1 (1977-08-09, 11 days before launch of Voyager 2) say this:
Failures in the Attitude and Articulation Control Subsystem (AACS) and Flight Data Subsystem (FDS) on the VGR77-2 spacecraft planned to be launched August 20 have resulted in a decision to interchange the first two flight spacecraft.
First launch is still scheduled for August 20, the first day of the 30-day launch window. The VGR77-3 spacecraft will now take the first launch date. Switching of the two spacecraft can be accomplished with minimum risk to the targeted launch date since the VGR77-3 schedule has always been predicated on the capability to support the August 20 date.
All testing and checkout of the VGR77-3 spacecraft continues at the Eastern Test Range (ETR), Cape Canaveral, Florida, with the pre-countdown test scheduled for August 8. Encapsulation in the spacecraft shroud is scheduled for August 9, with mating to the TC-7 Titan/Centaur launch vehicle at launch pad 41 planned for August 11.
The failed AACS and FDS have been returned to the Jet Propulstion laboratory in Pasadena, California. The spare AACS and an repaired FDS may be available for reinstallation in VGR77-2 at ETR by August 10, which could result in an encapsulation date of August 17
So what it looks like is that they built a total of three spacecraft -- one engineering model and two flight models. This is very common with space hardware -- build an engineering model first, run it through all the same tests that the flight model will have to endure, and basically use it as a pathfinder. Often the engineering model and flight model are of similar quality, and sometimes something happens to the flight model and they have to fly the engineering model. We don't generally build engineering models of the entire spacecraft any more, but it is still common to build engineering models of the more important parts (such as science instruments).
The terminology might be confusing -- why do we call the real thing which will be flung into space, a flight "model"? I'm not sure why, I just know that's the way it is. Just because it's a model, doesn't mean it's built of wood and plastic. It is the real thing.
From bulletin 3:
Three spacecraft were built for the Voyager mission. One, VGR77-1, was designated the Proof Test Model (PTM) and subjected to extensive testing in simulated deep space conditions to test the spacecraft design, construction, and durability. VGR77-2 and -3 were designated flight spacecraft and subjected to less arduous testing to save them for the real deep space conditions.
I think the proof test model VGR77-1 never left JPL and might now be the model in the Von Karman auditorium. The two flight models were then called VGR77-2 and VGR77-3 while on the ground.
Once in flight, it was always the plan to call the first spacecraft to reach Jupiter "Voyager 1". In order to get the encounter they wanted for it, they had to put it on a trajectory that launched second, but arrived first. The mission team had a choice of confusing the public for a short time near the launch by launching out of order, or confusing the public for the duration of a 5+ year mission by calling the first spacecraft to arrive at Jupiter and Saturn "Voyager 2". They chose the first option.
Due to the late switch, VGR77-3 got the first launch opportunity and became Voyager 2 in flight, while VGR77-2 got the second opportunity and became Voyager 1 in flight.
All of the Voyager mission status briefings have been archived by the Planetary Society.