I think this could be a 500cc F3 car converted to FJ at a later date.
Correct!
Is it the Ortschitt ?
No, because...
1959 Scampolo Wartburg Formel Junior
...this is the answer. The car started its life as a BMW-engined Scampolo 502 in 1950. Its builder, Walter Komossa, sold the car to Kurt Ahrens in 1952, who sold it to the East German racing driver Willy Lehmann in 1953. After victorious years in Formula 3, Willy Lehmann rebuilt the car as a Wartburg-engine Formula Junior in 1959.
I've found it only because I recognized Hamburg Prototyp Museum's floor
You can't be too careful when posting puzzles.