It is indeed the Steyr-Puch U1 prototype. I'm not sure if it was built in 1953 or '54, as the project was started in 1952 and the second quite different U2 was built in late '54.
This was the first prototype for a Steyr passenger car, two others were built before they decided to use the Fiat 500 bodyshell. this one had a motorcycle-derived two stroke engine that I believe was different from the motor they ended up using in the production car.
Well done MJW, another well-earned point for you - bad luck Allemano, hopefully some of mine will make it to the pros soon