Solved - much bigger than an NB Magnette, it's a 1937 MG SA Tourer Charlesworth, chassis SA1407, ".. first registered in England and by 1943, it had joined the Pacific Auto Rentals collection in California who rented unusual automobiles to Hollywood studios. It remained in the collection for 4 decades and was used in many feature films including Alfred Hitchcock's Rebecca starring Lawrence Olivier and Joan Fontaine, The Invisible Man with John Carradine, Cluny Brown with Charles Boyer and Peter Lawford, Love Letters with Joseph Cotton and Woman in Green with British Sherlock Holmes duo Basil Rathbone and Nigel Bruce."

Our collection of sightings.

Movie: Julia Misbehaves 1948
Car: 1935 or 36 MG NB Magnette 4 seat open tourer

Julia Misbehaves, made 1948, set in 1936 London and Paris.
Starring: Greer Carson, Walter Pidgeon, Elizabeth Taylor, Peter Lawford
Also starring: Cesar Romero, Nigel Bruce

The car that Walter Pidgeon drives when the four stars drive out for a picnic(with the bear) appears to be a 1935-36 MG NB Magnette 4 seat open tourer. The car is also seen briefly near the end when Walter Pidgeon and Greer Carson drive to a cabin looking for the eloping Elizabeth Taylor and Peter Lawford.

Sorry did not have the dvr running and haven't found an online clip of the car. The MG radiator emblem was very briefly visible as they piled in the car for the picnic trip. I believe the grille had vertical slats which would identify the car as a '35 or '36 MG NB Magnette.

Matt