I do believe you are right:
"The story of Rootes' Swallow actually dates back to the 1950s, and shows, yet again, how the British motor industry enjoyed a good deal of cross fertilization between the companies. The automotive consultancy company ERA, based in Dunstable, were commissioned by BMC to produce an advanced saloon car - with a view to challenging BMC's own designers (led by Sir Alec Issigonis) - which would have been intended to fit in the range above the Mini. The resulting rear-engined car was then sold to back BMC, who quickly dropped the idea, figuring that the Mini concept was just fine when scaled up. The rest, they say, is history...
So what is the link between this amusing concept and the Rootes Swallow? In 1959, ERA was sold to the Zenith/Solex concern, who had little interest in producing cars, preferring to concentrate on research into carburettors and fuel injection systems. The design team responsible for ERA's BMC proposal, therefore, concluded that it was time to jump ship, preferring to remain in the car business. David Hodkin headed that team, and they found a berth at Rootes, which at the time, was also attempting to re-invent itself.
In an exercise to challenge Issigonis' creative thinking, BMC management commissioned ERA to come up with a forward-thinking family car for the 1960s. This rear engined prototype was the result... dropped like a hot coal by BMC (and stored in their tunnel at Longbridge), the designers responsible for this car joined Rootes and set about designing a replacement for the Audax."
So, almost five years anon, I'll open this puzzle back up, with a point on offer if somebody can either prove this is the Rootes evolution of the ERA design OR produce a picture of what this cat looked like when Rootes was toying with it. Or, did the car's appearance remain intact?