Films made between 1928 and 1992 often contained a photograph of a woman (along with color bars) in the countdown of the film to assist film lab workers in color timing and tonal density when making prints. Projectionists often began films after the countdown frames, so audiences were unaware of these mysterious women (known as China Girls) but they were there....
The Northwest Chicago Film Society has a collection and describes them nicely:
"Their presence on the film (secretly sharing space with Hollywood’s starlets) is a fleeting visual document of the film industry’s vast off-screen labor pool. They remind us that every film made on film stock has a physical history, that each print we encounter in the projection booth, or projected onto the screen from the audience (or reproduced digitally on a laptop screen) passed through the hands of lab workers and technicians before it came to us."
