It's hard to assess how big the mystery car is. Without seeing more of the picture, it's difficult to judge how far in front of the Belvedere it is. The road also looks to be going uphill, in which case the perspective may make the car look less tall than it is in relation to its width.
A higher-resolution scan might help, but if the image is from a postcard, there may not be any more detail to be had. Enlarging the image, it looks as if the rear door has hinges on the right hand side, with the top one close to the bottom line of the rear window. A line across the tops of the rear lamps comes just below the number plate light. It has a plain rear bumper, and there's a door mirror on the left hand side which match with the 1100-103 Familiare.
I'm not saying that's definitely what it is, but I suggested it based on the visible details.
Has anyone got a decent picture of the back of a Moretti Giardinetta from about 1953? It sounds like a good possibility, but I can't find one.