Right now, I can't find a picture of an AMC Rambler or a Studebaker that has both the right shape and the right grill (or more accurately, the grill shown and the shape shown). The American Motors "American" of around 1963 comes closest with the grill, but that car is not as "rounded" around the doors as Agatha's car. Also, its name (metal script) is toward the front on the side, in front of the wheel well, not behind it, as shown in your first picture. Can you take a higher resolution picture of the side view so we can zoom in on the name? It appears to be just in front of the driver's door. But when I zoom with the current picture, it blurs too much.
The shape of the car looks like an early 1950s car before the cars all started looking pointed like rocket ships. That rounded look is the look of the Studebaker Lark, which I thought it might be. But the Lark has a small, enclosed grill, not extending all the way across the front from headlight to headlight.
What's baffling me is the 1950s "swollen" body look with the sleek 1960s grill. All I can think of is trying to get a close-up of the name on the side or a close-up of the hood ornament.
I can't find any American cars that still have that "rounded" look beyond the early 1950s. Were any British cars still looking like that? Wouldn't Agatha have a British-make vehicle? I know the British cars still looked more rounded than American cars by the 1950s-1960s. But I don't know the names of that many British make cars to look them up on Google Images.
??? I'm stumped.
golf