It is a white cardboard with bevelled edges placed between the frame and the photograph. The mat has a double function:
The American actor Steve McQueen on the circuit of the 24 Hours of Le Mans observes the rally for the needs of a film (Le Mans) that he will produce on the subject on June 10, 1969See all photos from Bridgeman Images
Bridgeman Images - AGIP
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Steve McQueen, Le Mans, shoot, movie