Research has brought up an interesting fact of which I was not aware. In late 1960, when the formula had just about died a death, Alessandro de Tomaso showed the press a Formula 2 car, Conrero-Alfa powered. It raced just once, at the Modena GP, where it crashed. It was later rebuilt as an F1 car for the 1961 season. Could this be it?
It does not seem to be one of the "bona fide" F1 cars raced by the team in the '61 season, and looks very new in the photo, just as if it was being shown for the first time. And the Formula Junior cars of the period 1960/61 from de Tomaso did not have that appearance.
PS - I have just found a (poor) picture of Roberto Bussinello racing his de Tomaso-Alfa Romeo in the 1961 Italian GP, and it has the twin bulges in the rear engine cover which I assume meant the motor was canted to reduce height. The puzzle picture, which is also not very clear, also shows two bulges in the same position. Does this suggest that this car is also Alfa-powered, and that it could be the mystery F2 car ?