It wasn’t the A-12 but a precursor. The A-12 didn’t come about until the late 50s. The Roswell incident happened in the late 40s.

There were a few experiments with supersonic jet propulsion around that time. There were also Generals who would gladly stoke the UFO flames to protect the secrecy of their anti-radar research.

I know this because my family was involved.

> It wasn’t the A-12 but a precursor. The A-12 didn’t come about until the late 50s. The Roswell incident happened in the late 40s.

> There were a few experiments with supersonic jet propulsion around that time. There were also Generals who would gladly stoke the UFO flames to protect the secrecy of their anti-radar research.

You're right with the timeline that the initial rumors didn't start with the A-12 radar tests, but they were reinforced by it. Initially, Project Mogul's crash of the balloon array triggered the rumors, but it's not as fancy and couldn't be interpreted as an "alien spaceship", even at the time.

https://en.wikipedia.org/wiki/Project_Mogul